This exercise is unusual looking but it serves a very distinct purpose. Most lifters lack the ability to properly centrate their glenohumeral joint. This deficiency is even more pronounced during the overhead press. The pizza plate press – using the flat side of a weight plate – will provide immediate feedback as to whether or not your shoulder mechanics and overhead pressing technique are dialed in.
If you fail to create 3-dimensional shoulder stability by depressing, retracting, and medially rotating your scapula and shoulders towards your spine, you'll dump the weight plate.
Even if you're able to avoid dumping it, keep your eyes on the weight plate. If it stays completely parallel to the floor while you keep your elbows tucked to the front of your body, then you know your shoulder function and overhead pressing mechanics are good.
If the plate fails to stay parallel to the floor, you'll then want to address your specific areas of weakness before you produce further dysfunction and possible injury.