The new doublestrike will work like Jax passive, you get a buff of 2 seconds of duration that resets its timer every autoattack and can get up to 4 charges. When the count is in 4, the next auto will proc the doublestrike and the buff will dissapear (this is different from jax because jax buff must account for R and passive as the same time).
I think that master yi late game will be a beast and a QQ maker, specially in low elos. I wonder if a bot yi+morgana lane could work... If he scales with player skill now he will be even more of a threat :D
Also this is a sad day for Alex Ich.