The difficult part is arresting your ascent so that you decelerate and stop precisely at some planned depth. The deceleration part is especially difficult with a drysuit (bag-o-expanding-gas) and heavy steel doubles (inertia). You need to coordinate how you dump from the wing AND the drysuit. And you need to do this so as to maintain a certain rate of ascent between stops--not too slow and not too fast. Dump too much too soon and you slow down too much or even come to a stop. Dump too little or too late and you accelerate and perhaps miss your stop. It's a difficult skill to master for many of us. Practicing this skill, many times I was unable to stop myself at 3m/10ft and popped to the surface. Now, throw in some task loading at each stop like
@Imla described and, wow--it is difficult to master.