The goal of this test is to jump as far as possible on each leg, aiming to "stick" the landing. The performance goal is to have the recently injured limb within 10% of the uninjured limb. Poor performance on this hop test (greater than 10% difference between limbs) can identify those at risk of re-injury.
Using technology from force plates, we can appreciate the ground reaction force (GRF) vector during the landing phase of the task. In the top left box of the first video, the vector is medial to the knee. If the vector were to move laterally outside the knee due to compensatory trunk lean or the knee collapsing in, there would be an increased valgus moment on the knee. This position places an increased load on the ACL. Uncontrolled dynamic knee valgus with a high acceleration rate is what ultimately can tear the ACL.