No-limits apnea is an AIDA International freediving discipline in which the freediver descends and ascends with the method of his or her choice. Often, a heavy metal bar or "sled" grasped by the diver descends fixed to a line, reaching great depths. The most common ascension assistance is via inflatable lifting bags or vests with inflatable compartments, which surface rapidly. The dives may be performed head-first or feet-first.
That is, the no-limits free diver descents by gravity, speeded up by heavy weights, and ascends by a blast of air filling a balloon-like thing, jetting him/her up to the surface. The whole journey from surface to 200 or so meters under water to back to the surface takes a few minutes. The diver holds her breath during the entire journey.
From the book, it is clear that the No Limits Free Diver trains extensively on land and under-water to attain competitive levels. However, it is not clear to me (a sometimes snorkeler and an infrequent but certified PADI diver) exactly what forms of athleticism are needed for this activity. (Not that I am considering it for even one picosecond!)
To put it crudely, and perhaps unfairly, the no-limits free diver is a passenger; they are attached to a line and taken down by gravity, not by their own efforts; and they ascend along the same line by holding onto (or perhaps wearing) air bags which have been inflated by a blast of air. They do, however, turn on the blast of air themselves. But in no-limits, they do no work to descend or ascend. They do, of course, have to get used to extreme pressure under water.
This is in contrast to constant weight free diving, where there is no mechanical assist in descent or ascent: the diver does the work on descent and ascent.
What does this question reveal that I do not understand about the athleticism of no limit free diving?