In its stock config, you just dip your left shoulder to trigger the ADV while simultaneously pulling the OPV cord. The ADV is in the T-piece, so you get known gas pretty much immediately, even before the loop is flushed. The ADV has a button on it you can use to manually inject, as well.
Some people replace the O2 MAV with a gas block and inject DIL on the exhale side. This configuration allows you to DIL flush more easily by pushing the gas block button and venting out the DSV, which comes at the expense of no longer getting known gas immediately because it has to go through the scrubber first.
The problem with the "classic" way of flushing by dipping the ADV lower than the OPV is that it does get dil to your mouth faster but is a much less adequate way of flushing the loop IMHO. We've discussed this before and despite Dave's penchant for the Venturi effect, flushing from inhale tee piece to exhale CL and OPV does not flush the head or canister. So I would say you are getting dil "at the expense of" a proper flush. Doing it the second way by adding a dil MAV to the exhale CL and letting the gas escape through loose lips at the mouthpiece will flush the exhale CL, exhale short hose, canister & head, inhale short hose, inhale CL, and inhale long hose to the mouthpiece. The only thing not flushed will be the exhale long hose between the mouthpiece and the exhale tee piece. And Venturi should certainly take care of that. Or using the MAV built in to the ADV in conjunction with a BOV like the Shrimp when turned to Bailout mode will force the added dil backwards through the loop and head and do a more complete flush before going out the OPV in the exhale CL.
As for what to do with a hypoxic loop, I'm exactly with Michael. Flip the shrimp to bailout mode, get the loop breathable with a dil flush, then go back on the loop and diagnose the problem.
But there are a few important points I'd like to make from the responses so far. These are just my opinions, and worth every cent you paid for them.
1. There is a big difference between a pO2 that is below your desired setpoint and a loop that is hypoxic (<0.18). One is a dive maintenance issue you resolve by squirting a little O2 in, no matter which CL. The other is a true emergency that requires an emergency response. I think some of the replies in this thread relate more to a "low pO2" than to a hypoxic loop.
2. When it comes to dealing with a true emergency, any of the replies that include the term "it depends" should be completely discounted. An emergency situation is no place to have to think, because your ability to think is most likely to be compromised by the nature of the emergency. Saying you would deal with the situation differently depending on whether you are deep or shallow, or depending on what dil you have, goes against the very nature of emergency action. Refer to Mark Chase's thread about his shallow bailout last year. In an emergency you should have one and only one, automatic, unthinking, reflex reaction. For me it is flipping the lever on my BOV. For others it might be a dil flush. No matter what, it needs to be ALWAYS configured to get you a safe gas at whatever your current depth is. No looking at depth gauges to determine whether you are above or below 6m. No checking to verify cells. That will all come later. Getting a breathable gas through a single reflex reaction is paramount.
3. I have no idea why the Meg was designed with the O2 add on the inhale side. That design troubled me for years. But the fact is that thousands of dives have been done safely without it ever causing a problem. So it probably doesn't matter one bit which CL gets the O2 addition. Even taking a full breath of pure O2 at depth will not be instantly fatal. Your tidal volume is not your full respiratory capacity. And OxTox is dosage related, involving both pO2 and time of exposure.
NOTE: I made this post and then half of it disappeared for some unknown reason. So if my re-creation lost a little in the process, I apologize.
