Best to do this on flatter section of the t-bar. You need to boost yourself forward (clearly seen in 1st vid), I do somethin similar in the guy in the first vid, pull back on the top of t- and scoot forward, to create some slack. you'll stop& turn skis and wait for tension and then pop and look over shoulder. I find it helps to do a couple boost/scoot forwards first just to get the pop timing right.
double flatspin was siiick!