These bushings sandwich between the transmission mount and the front crossmember. Yes, these bushings support the transmission mount, which itself is a rubber bushing. Only the Subaru engineers understand all this.
Anyway, the Kartboy version is stiff polyurethane, which will reduce unnecessary movement and free up power to send to the wheels instead of pushing your tranny around.
Installation takes a bit of work -- excellent picture-enhanced instructions are available over on Scoobymods. Set of two two-piece bushings.
Note that like any bushings, these do have a finite life. After about five years, inspect them periodically; plan to replace them once they lose structural integrity.
Fits all 2002-07 Impreza/WRX/STI models. Forester and Legacy too! Does NOT fit 2008+ cars.