FLCH...
As for the OP...difficult to know what Tech level to pitch the reply at if we don't know the tech level the poster is working at:
Simple answer "FLCH 101 for some Boeings" - using FLCH tries to give you the required change in height in approximately two minutes and at a fixed speed..
..However if the increment is so big the system can't achieve the change in two minutes it will climb you at climb power to get you there as soon as it can, if descending it will descend you at best rate, fixed speed, idle power...
Probably most used for descending/climbing when you aren't trying to follow a planned/published vertical profile, or you just want or need to get the level change done promptly (e.g. an ATC "descend now").
If the Op wants something deeper then I'll leave it to the grown ups.