How do I use Tech Armor?

Tech Armor is a two-stage power, of which there are several in this game. (Fortification and Defense Matrix come to mind as being very similar) It's a little confusing though since I think in previous iterations there weren't any powers that worked this way.

The first time you hit the button to activate the power, damage taken will be decreased but your power cooldowns will also lengthen. The second time you push it, it detonates, doing damage to enemies around you.

This class of power is really intended so that you can soak up damage while you get up close and personal, and then get a quick damaging attack when you go in for the kill.

In my experience, there's kind of a faint glow that surrounds you when you've got the armor activated. It's kind of easy to miss, so I've at times set it off when I was trying to activate it. If you're playing on single player, you might be able to go by the powers that are active on the power wheel. The power cooldown penalty is also noticeable enough that it can be an indicator.