simple answer: any portion of a dive (pre, during, or post) where you do not have (1) understanding, (2) input, (3) equal say in what is going on, outside of the controlled training realm.
essentially, its putting it in someone else's hands to manage
I would add to that definition;
It is also any dive that you could not safely complete without assistance.
The 'trust me' element is where you rely on the greater skill, training and experience of another diver to ensure safety where your current level of competence is not sufficient.
It requires an
abdication of personal responsibility for your own safety to a third-party.
There is a distinct difference between a 'trust me' dive with a dive pro, or any other more experienced divers, and a training/mentoring dive. The primary difference is that in a training/mentoring dive, the supervising diver will ensure that you grasp the knowledge and skills needed to complete all aspects of the dive
before you get in the water. In contrast, a 'trust me' dive tends to consist of nothing more than
"follow me, you will be ok, I will look after you".
What about diving with a guide when you are new?
As a trained, qualified, certified diver, you are responsible for your own safety - with the added safeguards of adherance to the buddy system and, at times, the supervision of a dive leader. Nonetheless, those other people are no more responsible for
your safety than you are for theirs.
With most scuba agencies, you are given the advice to dive only within the limitations of your current training and experience. That means adhering to depth recommendations, not entering overhead environments and diving within water/weather conditions with which you are familiar. Basically... it means you are diving in circumstances for which you are trained, skilled and educated in - and this is a major factor in ensuring your safety. It also means
using and
applying the skills, techniques and knowledge that you had been provided with during your training. This would not be a 'trust me' dive, because you
should be qualified for the diving that you are doing and you will have the right tools to complete that dive.
Diving under the supervision of a dive leader
does not excuse you from these safe diving practices; especially the responsibility for ensuring your own safety, for diving within your recommended limitations and for
always applying the skills, techniques and knowledge that you aquired in the process of your certification. Those people who
do view having professional supervision as an option to 'opt out' of responsible decision making, abdicate personal responsibility and not bother to apply what they had previously been taught - are the ones who make 'trust me' dives.
Sadly, too many scuba industry 'pros' actually encourage divers to make 'trust me' dives. Ego can cause them to over-estimate their ability to safeguard divers, commercial pressures (
the desire to please) can cause them to make bad risk assesments applicable to their customers, sheer negligence can make them act as bad role-models and encourage unsafe diving.
There are numerous negative consequences that result from doing 'trust me' dives. In the worst instance, they can lead to loss of life if things go wrong. At the least, they serve to encourage divers to over-estimate their own capabilities - because divers can grow complacent about completing dives where they
actually are very reliant on others should things go wrong...