The Wimberley AP-901 Sidekick Shifter Combo moves the base of the Sidekick away from the ball head just enough to reposition the center of gravity of the lens, moving it closer to the center of the ball head, thus allowing the lens to balance optimally. The Shifter Combo is required (or is an alternative configuration to using a replacement foot) when using specific lenses with the Sidekick. If the Shifter Combo is not used where recommended, the user will sacrifice performance.
A few larger lenses may also require the use of a quick-release replacement foot (to reduce the lens profile) in addition to the Sidekick Shifter Combo; however, most lenses that require a Shifter Combo will work well in combination with a standard QR lens plate. The Shifter Block and Spacer Plate combination of the AP-901 attaches easily to the bottom mounting base of the Sidekick using pre-drilled mounting holes.