These occur in highly competitive auctions, such as:
N E S W 1 2 2 3 X
The double is used as an invitational bid. Partner is expected to either bid 4 with a maximum, 3 with a minimum or pass for penalties. A direct bid by partner of 3 is not invitational--- just competitive.
NOTE: The maximal double is used in its original form only when the opponents have given you no room to make a game try. Thus, in 1-2-2-3; 3 would be the game try (nothing about hearts) and double would be for penalty.