system
Given the following post, respond as if you're commenting on Reddit:
human
[X-Men] Quicksilver's powers

Now this is in regards to Quicksilver's powers in Apocalypse, but in general for speedsters, I can understand them being able to control their running, but how can they control the speed at which they fall? Like they can watch people fall in slow motion, because their mind is going faster than normal time, but their own falling speed can't be stopped from becoming lethal.
gpt
With most powers that defy the physical laws of a normal human's body, they have secondary powers to cope with literally everything like that.

In his specific case his mutations actually adapted to that of a person who'd be able to move at those speeds. No real hand waving. He has tougher skin and stronger bones to deal with the impact of running, and of course falling at those speeds. He has a coating over his eyes that allows him to keep his eyes open at those speeds. Tougher joints and ligaments to prevent his body from just separating, and so on so forth.