I'm seriously asking for information now. Why does
@DiveGearExpress's suggestion seem so onerous to you? Are you so in the zone that a casual exchange will throw you off the game? Is it like asking Tiger Woods a question in the middle of his backswing? Why does such a seemingly innocuous suggestion provoke such a reaction?
The real challenge is that rebreathers are a lot more complex than open circuit. It starts at least the day before when you build your unit; did you fully follow the checklist for the hour plus process? Everything from analysing your gas, filling scrubber, checking cells, checking batteries, positive and negative checks, making sure the unit's built correctly, checking the gases are programmed in correctly, etc.
The pre-dive checks take ~10 mins and include checking that all's plugged in, all's turned on, you can breathe, the computers are working, then a 5 minute pre-breathe which will validate the scrubber and that you're not "feeling funny". Basically chill before jumping.
The real things to check on a rebreather diver is pretty much the same as for OC: is the drysuit connected, gloves, hood, mask, stages, cannister torch, any nefarious dangly bits...
It is extremely good practice for the rebreather diver to stand at the gate and puff some gas in the wing and properly look at the PPO2 on the wrist-mounted computer in a way that other people can see that this check has been done:
- Will I float?
- Can I breathe?
Everything else can be dealt with on or under the water.
A quick point about OC divers. Tech OC divers** differ from recreational divers*** in that they've got the ability to resolve gas issues by themselves. The classic DiveMaster person is trained to look after the recreational divers who are frequently novices. In fact DiveMasters have to assume they're novices unless otherwise ascertained.
The rebreather 'knee-jerk reaction' is about the fact that rebreather divers would have put in a lot of training to work around the complexity of the unit. These aren't novice OW divers, but people who've got a demonstrable degree of skills. The classic DiveMaster has none of those skills (assume we're talking about your run-of-the-mill OC DM here). Whilst being concerned that the oxygen is switched on is a nice thing, it is just one of many many things that can go wrong on a rebreather.
** meaning diving with a twinset / sidemount and having practiced shutdowns and the like
*** meaning diving with a single BCD where your reserve is someone else's octopus