I am a prep coach and there is some truth here, but the part about sodium/water depletion and then diuretic use piled on top is not only a path to death, but also a path to looking like shit. The key is being in top shape 10 days out and not doing crazy last minute stuff.
You look your best if you keep drinking water until the afternoon before a show (I actually keep a little going until the evening) and don't completely cut sodium. You are then in a position to let diuretics work their magic.
And you will actually look your best by keeping diuretic use moderate. Yes they are very dangerous, but not if used properly. You start on an aldosterone blocker, titrate up, then a thiazide, titrate, and don't touch a loop until close to the show. Its the loops (lasix) that will kill you but I only have my people take one, maybe one and a half tablets max. Also, I re-introduce sodium at the same time a loop is used. The key is to stay conservative and keep use to as little as needed. And I come in with shredded glutes this way. I felt the the Olympia guys were not very conditioned at all. The real problem is you have to be 100% in shape 10 days out and not try and do any last minute crazy stuff to compensate for being out of shape. That is a problem I am having right now, I have a 280 lb guy who thinks he is too lean and that diuretics will solve his problem. The fact is he has to much fat on his stomach. Diuretics only work if you are already in shape.
If you are doing crazy last minute stuff, depleting and loading, cutting water for days, using a ton of diuretics, you are doing something wrong. You should be completely shredded (and flat) 10 days out and then gradually carb up. Diuretics are just the finishing touches.