The Jump Reverse is used to show two things -- four card support for opener's suit, and a singleton in the suit bid. In Hard's 2/1 system, this is not game forcing; other systems make this game forcing.
This system distinguishes between two types of mini-splinter bids. The jump shift in a suit below opener's shows a singleton and is game forcing, while the bid in a higher suit is game invititational only. This compares with the Over Jump Shifts, which does not necessarily specify where the shortness is located.