While general strength from exercises like the squat is important, it is not going to be the perfect indicator of a person's jumping ability. A person's jumping efficiency is going to really be the difference between a strong person with a good vertical and a strong person with a great vertical. Jumping efficiency is basically how fluent one can execute the movement. The two footed vertical jump in essence is a triple extension of three joints: the hips, the knees, and the ankles. By efficiently using all three of these joints, a person will noticed added power and hang time in their jump. The extension of the big toe joint is kind of an underground secret to that extra inch or 2. This quadruple extension can be noticed is almost all great jumpers. Check it out next time you watch a basketball game.
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