Just a guess, Im thinking it's a slight nudging, just enough to change the path of the ball.

have you tried different pitches, that should change the path somewhat

I'll be danged if I can remember where I read this, but I believe the "Williams twist" (that's what the review at ipdb says, although "wrist" would make sense too) is when the ball comes down the left outlane and hits the lip of the outhole enough where it bounces back up to the right flipper. I also have a Gigi and this is a real game-changer. I've played a few more Gottliebs that do this and it must be something with the apron of the period. So why is it called a Williams twist? I don't know (and I don't know 100% if what I said is accurate anyway). Maybe I haven't played enough 60s Williams games.

Betting this technique was developed and named by a premier old timer EM player, Sam Harvey,
Who had an affinity for early 60s Williams games, that were designed to have lots of nudging action
for best playability.