The Wimberley AP-900 Sidekick Shifter Block 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 Block is required (or is an alternative configuration to using a replacement foot) when using specific lenses with the Sidekick. If the Shifter Block 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; however, most lenses that require a Shifter will work well in combination with a standard QR lens plate. The Shifter Block of the AP-900 attaches easily to the bottom mounting base of the Sidekick using pre-drilled mounting holes.