These drills separate jumping into its separate components, the up part, and the down part.
The key to both of these drills is landing with your knees no more bent than when you took off.
First: jumping up onto an object
The value of jumping onto objects is that is minimizes the stress to muscles and tendons because there is much less excentric contraction than if you were landing from the same hight. Additionally, having an object to jump onto gives good automatic feedback. Either you give max effort and land on the platform or you eat splinters. How’s that for incentive?
Ideally you want to pick an object or box that allows you to land with your knees less bent than what they were when you took off. You could ague that this picnic table is too high for me. I land with the angle of my ankle-knee-hip about 90 degrees but when I take off, that angle is closer to 120.
Another alternative is the step to box jump
This helps practice transferring lateral momentum to vertical momentum. In this jump I get up to the table a bit easier and the ankle-knee-hip angle is approximately the same as it is upon takeoff.
Second: jumping off of an object
Jumping off of an object allows you to work on your landing. This is a part of having proper progression in you plyometric training. If you cannot land properly, then you are not ready for higher intensity plyometrics. Working on your landings allows you to train your tendons and muscles to absorb force. If you cannot absorb the force of landing, then you will not be able to quickly generate force after landing.
As in jumping up, you want to pay attention to the angle of your ankle-knee-hip. If that angle is less than 90 degrees, you need to jump from a lower height. If this is true of your landings from your max height jumps, you aren’t able to absorb the force of your own landings and you need to do some strength training before venturing into high intensity plyometrics. Also have a friend check o see that your legs remain relatively parallel to one another and that your knees are not turning in. If they are, you should get yourself on an ACL injury prevention program asap.
Why separate the two components?
Why not just jump up and down? Certainly you can put these two pieces together and do tuck jumps. But separating the up and the down components of the jump does allow you to focus better on each component and the platform gives you feedback about the height of your jumps. In tuck jumps, you have no way of knowing if you are repeatedly jumping as high as you can.
you should also read the latest Ultimate Athlete Expert Panel on the topic of Getting Faster, Jumping Higher