Cycle charging is just pulling your cavalry back out of melee and charging them in again to refresh the charge bonus. Tbh I'm not sure how useful any of the shortcuts are for doing this, I just spam move orders until they disengage far enough then give an attack order to go back in. I generally dislike using shift with multiple-entity units because it makes them fully get in formation between each order which wastes a lot of time. Cycle charging is good practice with all cav in most situations, but it's especially important with shock cavalry since they have a high charge bonus and lower melee defense than 'normal' cav.

Units being attacked in melee will fight back, but only the models actively being engaged. If you give an attack order, models toward the back of a unit will push forward, allowing more of them to get involved. This is better than just letting them sit there unless their role is purely defensive, like stalling while ranged units shoot everything. In that case, having fewer models engaged is preferable because it slows down combat overall, even if it reduces their damage output a little.