I'll try to answer that with all my knowledge of newtons laws and my wee amount of experience with the game.
Since realism is so important for simulation when you order a crash dive your boat gains a much more momentum as it goes down deep under water with dive planes set to maximum and the crew is making every other effort as quickly as possible to get the boat under water.
To counter it's undesirable side-effects, you have to give new orders for a shallower depth much earlier before you gain even more momentum and go deeper. If you're crash diving and your current depth is 50m if you give the order to go up to 25m it will take much longer for the boat to respond to depth adjustments as compared to the usual, depending heavily on the speed of your boat. For this purpose, some people start "spamming" the (say) 25m depth command at 50m if you don't want to hit something like 170m when depth adjustment actually takes effect and the boat starts to rise again.
Additional things to help control the rate of descent is to set engines to full reverse and blowing the ballast tanks.
To the more knowledgable people: AMIRITE?!
