I don't understand how to activate power mode (for purposes of multiplayer).

You use power mode for running and power jumping.

So it is automatically activated when you are NOT in stealth/armor mode + you are running or jumping?

What are you saying exactly????!!!!

What are you saying exactly????!!!!

How do you unlock power modules

You don't really activate it.
You just use it when you sprint, high jump, and slide.

You get module unlock tokens for each specific one like cloak or power, like for example is you spend time in cloak or get kills with it to a specific point, you unlock a stealth module, if you use power just run and jump for sometime and you will get the power module, and for armor mode just spend time in armor mode like same with cloak and then you unlock the modules.

It helps if while running and jumping and sliding you kill someone soon afterward..