I think it is more like his subconsciousness/body/power activating to give him a power if he needs it and has no other way to overcome the situation. Hence why his power activates rarely and only after a great stressor.
In the first episode, when MC had no training/conditioning, he (and this cute new girl), were saved from being pancaked by his power, giving him a temporary weak shielding to halt the flung car.
I think it didn't activate when Miss Powell threw a desk at him because MC's body had already undergone serious conditioning/training at that point, so when the desk was flung, his body/subconsciousness/power could tell it wouldn't seriously hurt him, so no power was needed. Same thing with Lovestruck, he never got any noticeable power because he didn't actually need anything to defeat the situation as proven by him beating her.
Then again, MC also never noticed he had night vision, so who knows what his ability actually did for him in other situations.
Reason why it would activate in the most recent episode?
As Mrs. Powell said, powers can be activated by heightened stress (re: Adrenaline), hence the desk experiment.
So when Chrys phased him both through the ground and out of his VERY protective suit, it probably caused him to freak, adrenaline spiking, activating his power. When activated, it realized the only thing MC couldn't do on his own in this situation was see, so it corrected that. I think the same thing would happen if MC drowned, he'd develop gills. Currently, his powers are weak, so it gives him just enough to squeak by.
Its reactive evolution aka adaptation
A stupidly powerful ability, but also limited because until he has to adapt to something, he's just a dude.