The servo saver allows the wheels to turn without turning the servo - a way to prevent servo breakage on impacts. This is the one you should use:
http://www3.towerhobbies.com/cgi-bin...?&I=LXJHN8&P=7
And yes, it goes onto the servo output gear.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz