Looks like you got it! For everyone else: a slider animation controls the IK mix. When the slider's bone enters the range, the IK is turned on, when it leaves the range it is turned off.
These setups are definitely more complex than just setting keys. Mostly we are just exploring the new tools. In the real world you would have some goal, then hopefully you use the advanced setups only when warranted. We see far too many projects that have literally 500 or even 1000 constraints, and that's before sliders and the new transform constraint. Please don't do that!
If you've missed the last few updates, check the changelog. The update today was very important, I posted about it here:
https://esotericsoftware.com/forum/d/28555-4317物理约束出现奇怪的形为/20