In Leopard, when you hold your mouse on a stack, a menu pops up like a control click. Now when you hold your mouse on a stack, the actual stack pops up. BUT when you proceed to move you mouse on an item in the stack (while still holding the mouse), a grey selection box appears around the item. Once you let go of the mouse button, the item selected by the grey box will open.
Now, imagine your finger doing this action on a tablet – holding and dragging to the item you want in the stack. This saves time by not having you lift your finger off the screen to select the item you want. This motion is similar to the one on the iPhone keyboard, where you can tap and hold on the caps lock key, and then drag your finger to the letter you want which gives you the capitalized letter.
Pretend there is a mouse cursor on the following pictures. OS X removes the cursor from screenshots.
Clicking on a stack and selecting:
Clicking and holding on a stack and selecting:
Now, imagine your finger doing this action on a tablet – holding and dragging to the item you want in the stack. This saves time by not having you lift your finger off the screen to select the item you want. This motion is similar to the one on the iPhone keyboard, where you can tap and hold on the caps lock key, and then drag your finger to the letter you want which gives you the capitalized letter.
Pretend there is a mouse cursor on the following pictures. OS X removes the cursor from screenshots.
Clicking on a stack and selecting:
Clicking and holding on a stack and selecting: