In the case of a two-faced line, the couples will be turning 1/4 (90 degrees) around the hand-hold between the two couples (exactly half as much as for a Couples Trade).
In the case of a one-faced line, the couples will turn toward each other. Each couple will turn 1/4 (90 degrees). The couples will then take right hands to form a two-faced line.
Take care not to confuse Couples Hinge with Partner Hinge. In a Couples Hinge, the units are couples -- the dancers in each couple stay together, holding hands. In a Partner Hinge, the individual dancers are the units -- the two partners who make up a couple are doing a hinge with each other.