Now, if you want to raise your hand in front of your face to look at your gauges/computer without venting gas, then maybe you want to keep those gauges on your right arm. Hence light on the left. Also, if you have a long hose and want to donate it, then it's done with your right hand, isn't it? Your light is best kept away then. This is true both for bm and sm.Most of my muscle memeory came from diving backmount, so I still keep everything on my right hand, while only having my light on my left
- I'm sure there are other diving styles; this is but my preference
Both cylinders are very much primary.and I use minimal movement with my right hand to inflate my wing or suit. While I dump air with my left hand. So it was logical to have it split between left and right like that to a (primary and secondary cylinder).
In sidemount diving you need to switch regs every X minutes or every Y psi to make sure the pressures remain very close to each other. More buoyancy on one side does generate torque and fighting that strains your one leg and this can lead to muscle cramps. Yeah, been there. Didn't watch my SPGs and got leg cramps and it was under solid ice. You can guess if that was my happy place. Hint: breast strokes are a valid method of propulsion in sidemount diving
Donating a tank might happen in a restricted cave environment without long hoses. It has surely happened.since I donate the right tank (secondary), my primary buoyancy is my wing, so I figured it's supposed to be on my primary tank which will never be donated. And I can attach both my drysuit and my wing as needed if I have to donate my left tank.
If you dive dry and wear steel tanks to replace some of the lead (or to withstand the hardship of sump diving), then donating a tank would be unwise. Long hoses are great.