By Mike King
Tire pressure definitely does play a roll in learning to hop. You will find it easier to start with lower pressure to learn how to snap the wheel. By snapping the wheel, I mean to use the tire compression to help get more height. I started with really low pressure in my 24x3" Gazz (around 20 psi) because lower pressure gives you more bounce with less effort. It will also really let you feel how soft the tire is and you will learn quickly when to push the tire down and then pull the uni back up. If you use a high pressure to start with, it is very hard to feel the tire compress.
So, to snap the tire, you want to stand up slightly on your legs (actually, just lifting some weight off the saddle) then push the tire down hard before your hop to compress it and then pull up on the seat and jump with your legs for your hop. I have learned now to compress the tire by actually bending down my upper body quickly so that the downward weight helps compress the tire and then when you spring back up with your legs and upper body, you can get more height.
As you learn this technique you will likely soon get to the point where you can bottom out the tire to the rim and feel it on the ground. When you get to this point, you should increases the air pressure bit by bit as you continue to increase the quickness and pressure of your snap. This will train you to speed up the push down and snap back up. The higher the pressure, the faster your snap needs to be to bottom out the tire and it will help you to get a little more height. I typically run between 26-30 psi for trail riding and at least 30 psi for trials since edges and rails can bottom out to the rim easier than the ground.
The idea of the snap is also to let the uni come up on its own underneath you, instead of pulling the uni up which takes from your body's height. If you can learn to snap the wheel up for the height of the hop then your jump is only to lift your body above the uni, not both.