Thanks for all compliments and suggestions!!
@Lungfish:
The nitrogen build-up in beginning freedivers is too little to cause problems.
Purging is not for beginners and in my opinion it's the same as hyperventilating which should be avoided anyways!
Deepwater black-outs are really rare and not stuff for beginners.
The vacuum effect as you call it (never heard of that term) is described in the shallow water black-out section.
