In Stack 'Em High Poker, when holding 2 or 3 aces, you are awarded several additional hands with the same cards. Any drawn ace is "sticky" and will duplicate and improve all your remaining hands. 3 aces become 4 and now you're just sweating the kicker. Extra hands expand into the top box creating additional excitement and possible bonuses.
Which isn't quite right as in DW versions it is the deuces that can stack.
It's pretty easy to figure out, but keep in mind for it to "stack" you can only use the aces, (or deuces).