Shear adhesion makes use of microstructures to activate adhesive forces between a surface and the gecko's toes when shear force is applied in a certain direction. That's force applied in parallel direction to the surface. Removing shear or pushing in the opposite direction will deactivate the adhesion. I'm only getting this info from an engineering paper by Hawkes et al. about gripper design, so it may be a bit different with geckos.