The Fork/Palm Stack is a position where two balls are held in one hand one on top of the other, one in a fork & one in the palm (hence the name). This position is used in several other Fork tricks which is why it gets its own entry.
Pick up two balls & hold one in each hand. Turn one hand over to face palm down, grip the ball using only your thumb & middle finger & extend your ring & index fingers to form a fork. Throw the ball from the other hand & make a Fork Catch so that you are holding two balls one on top of the other in one hand.
Practise throwing one ball from hand to hand making Fork catches whilst holding a ball between thumb & middle finger in each palm.
This is very good practise for The Twist, The Toss Up, The Two Ball Fork Exchange & The Three Ball Fork Exchange.