Action Doubles
An Action Double is used by responder in the balancing position, typically after partner has bid, opponents have overcalled, and the bid is passed around to responder. When responder has bid to the extent to his/her values, responder cannot rebid without either (a) a long a good suit, (b) support for partner.
With game invitational values and neither of these two types of hands, responder may double in the balancing situation to force the bidding forward.