As the guys said... yes is when you rub the side of your thumb rigth after picking the string it's almost like rolling the pick away from the string rotating your fingers and in the proces you hit the string again with your thumb, and to achive better result wiggle the string wih your fretting hand.

I read a funny story bout Trivium I think... this guys played a Black Label Society CD and drink a beer every time Zack did a pintch harmonic... you don't need to be a rocket scientist to know what happened.

Keep trying... pintch harmonics are a lot of fun and add intresting stuff to your playing.

Rock on bro.