What takes place in power mode?

Yes.
If you engage B on a long descent, (over 600 foot vertical drop) it uses engine braking to lower the need for friction braking, (keeping the rotors cooler) and regenerative braking (keeping the battery cooler)

If you use it on a short descent, you will not get the full amount of regenerative braking, so you get poorer fuel economy.

B has almost no effect if you are not braking, although you can't engage Cruise Control in B.

I have not been over 95 MPH, but my experience is that you can't fill the battery on level ground even 95 to 0.

 

FYI, I can confirm that cruise control engages B mode automatically as needed. You won't see the indicator on the MFD say anything, but if you listen on a long downhill run, when the battery icon is full and regen cancels out, you'll hear the engine revving higher and higher to maintain the set speed.