Suicide strap rig

I recently acquired a used set-up that had the octo spare air on an extremely short hose with a suicide strap on it. It looks like the previous owner had it set up so that his octo was attached to his neck- the hose is so short, I can't see it practically being offered to another diver in an out-of-air situation. I was wondering why anyone would want to set their regs up that way? The primary hose is of average length.
Thanks!

His way of diving was to give his primary and put the octo in his mouth for breathing on a shared air emergency.

The diver retains the AAS for their own use. If their buddy needs air they will donate the regulator that is in their mouth (primary reg) and then switch to their AAS. This is typically done when the primary regulator is on a long (5-7') hose.

Not sure what you are calling a "suicide strap"? A necklace bungee?

The more well known term is Suicide Clip and refers to an actual clip.
Suicide strap is a new term for me, I've never heard it before.

It's a different style of diving. The idea is to use the Octo as a backup. Tying a loop necklace onto your regulator means you will always have a backup in the same spot everytime, no slipping/dragging octo. If it's set right you can even deploy it without using your hands. If you get turtled in a surf zone you have a regulator right below your chin.

This style of diving means you will have to donate your primary in an OOA situation. This means you are handing off a regulator that you for sure know is working and not stuffed with sand or free flowing.
Since your backup is near your neck it's hard for that part to get stuffed with debris, and in the off chance that it suddenly fails to work (even after a buddy check top-side) you can then go to buddy breathing.

One of the only differences I saw with the SSI vs PADI OW course was the air-share method. In PADI it seemed to be that you swam to the diver needing air, and let them take your occy.

The SSI method was to swim up, give them your primary, get them sorted, breath off your occy, then swap once the other diver was sorted. In other words, you as the diver giving out the air, was in control of how and when. And your initial donor air source was your primary, not your occy.

Whereas the PADI system was letting the panicked diver drive the air share event.

Having now seen what happens when a diver without air underwater panics, i see the SSI system being far smarter.

But I wonder how long the hose must be on the primary to use that as the sharing reg? Is it longer than normal?

It is uncomfortable to share gas on a standard 24" regulator hose, although it can be done. The reason octo hoses are usually 36 or 40 inches is because it is much more comfortable to use that length. If you are going to put your primary on a hose that long, though, you may have to use a right-angle adapter, because if you run the excess hose under your arm, it will come up at an odd angle to your mouth, and the regulator may not sit nicely. If you use a five foot or seven foot hose, the wrap behind your neck means the regulator will sit as it normally does, and no adapter is needed.

When I first did my OW course, I was taught to donate the secondary regulator (Octo) & retain the primary. I was fortunate enough to switch to donating the primary & switching to a bungee'd secondary early, so the transition was smooth. My LDS (SSI) teaches to donate the primary & switch to the secondary. The reasoning is that it will work with any equipment configuration. I have seen some divers that are trained (& it is ingrained) to give the secondary & keep the primary, switch to an inflator hose integrated air source & are at a loss a what to do, until given instruction & practice with it. On top of that, with the few OOA situations I have been in, it has yet to be text book. I have yet to have a panicked OOA diver ask me (by hand signals), "I am out of air, could we share air?" Generally, it is on par with getting mugged, by having the reg I am breathing off of ripped out of my mouth unexpecetedly. These have not been my buddy, but someone who saw me as their closest air source. Where their buddy was,... I have no idea. Not that it bothers me (except for the suddeness), I just pop the secondary regulator sitting below my chin in, take control of the situation & get us both safely to the surface. A panicked diver is likely to go for a regulator they see working. I have NEVER had a secondary regulator come out of a bungee necklace, unless deliberately pulled out,... unlike the various Octo holders I've seen. In fact I have seen many times, secondary regulators that have fallen out of these various holders, dangling (a huge pet peeve of mine), being drug over the corals, through the mud, sand, silt & snagging on anything it can find. To me that is much more suicidal than having the secondary neatly tucked up under my chin with its necklace, within easy reach. The only argument I have heard against donating the primary & taking the secondary for yourself, is the fact that for a very few seconds you have 2 divers without an air source. IMO, if a diver is so uncomfortable that they can not be without an air source for about 5 sec or so.... they really have no business being in the water to begin with. My recrational set-up is a primary regulator on a 36" hose & the secondary on a 22" hose. My technical set up is the Primary regulator on an 84" (7') hose & the secondary on a 22" hose, both secondaries are bungee'd.

OzGriffo:

One of the only differences I saw with the SSI vs PADI OW course was the air-share method. In PADI it seemed to be that you swam to the diver needing air, and let them take your occy.

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There's an emphasis on who the skill is being taught to (donor or reciever).

At OW level, PADI teach the receiver to be pro-active and attain the AAS. They don't make any assumption about the competency of the donor, thus the OOA diver needs to have that capacity to ensure their own survival by locating, securing and breathing from an AAS.

At Rescue Diver level, PADI repeat that skill, but with a new emphasis on teaching the donor how to conduct an AAS intervention - being proactive in identifying and actively assisting an OOA diver. At this level, the diver is taught to approach and deliver their AAS.

OzGriffo:

The SSI method was to swim up, give them your primary, get them sorted, breath off your occy, then swap once the other diver was sorted. In other words, you as the diver giving out the air, was in control of how and when.

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As mentioned, it is all about the focus of who is being taught in the drill.

You can either teach divers to be proactive in donating their AAS or you can teach them to be proactive in ensuring their own survival by locating/securing an AAS from another diver.

PADI save the donation focused skills for Rescue Diver level. At OW level, they teach divers to help-themselves. There's some logic to that.

OzGriffo:

And your initial donor air source was your primary, not your occy.

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That's not an SSI standard. The exact mechanism of AAS drills is determined by the instructor and/or the configuration used. That applies to both SSI and PADI.

OzGriffo:

Whereas the PADI system was letting the panicked diver drive the air share event.

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Again... the focus is on self-reliance.

Letting the donor 'drive the air share event', from the most basic level of training may deprive divers from the sense of ownership that they should develop towards their own safety.

OzGriffo:

Having now seen what happens when a diver without air underwater panics, i see the SSI system being far smarter.

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There's pros and cons to both. Personally (as a PADI and SSI instructor), I prefer students to develop both options - to be a pro-active donor and also to be pro-active in preserving their own safety as a receiver.

Theoretically, If I was limited to one approach, I'd always hedge on developing individual self-preservation skills that didn't promote reliance on others.