Hmm, have to disagree with Bob Simpson there (yes, i'm arguing with a former test players knowledge of the game....... really out of my comfort zone now).
Taking a ball on the up...... thats got to be the most illogical way of catching i can think of.
Firstly, the physics of it. Taking a ball on the up rather than the down means you must be standing nearer. Which means less time to judge, position and react. Sure the great keepers probably had the best reactions. But just because you are capable of reacting faster, doesnt mean you have to push yourself to react faster each time. Means more errors.
Now onto the ergonomics.
Taking the ball on a down arc is far comfier and less strainfull on the body. Holding your hands cupped at 45 degree's upwards, legs close together (better springing position than spread wide, learnt that from being a goalkeeper), knees slight bent. In this position letting a ball drop into your hands allows your body to follow the ball's path, bringing it down in your hands and slowing it gently, will result in few drops if concentrating properly. Ideally you should be aiming to start a catch about waist height, 2 inches infront of the body, finishing about 4 inches behind your knee, pivoting the body around the hips smoothly throughout. This'll bring the ball slowly to a halt, under control without diverting its path, thus making it unlikely to come out of the hands. Added bonus of this is the ball will NEVER leave your eyeline (if your focused)
Now catching on the up, for one you cant hold your hands in the position above, as the ball will just wrap your knuckles. Which means your hands are going to be cupped at least horizontally, mebbie at an angle pointing down. Just try cupping your hands 45 degrees up to the side of your body, then moving them to a horizontal, possibly slightly pointing down position. Firstly you'll find your elbow is in your stomach/ribs, and your shoulder will be strained.
Following that, tracing the balls trajectory is a damn site harder to. When catching on the down, its a smooth motion if you pivot the body. Following it slightly on the up means its pulling your arms up and behind you. As i've already pointed out, your elbow is already stuck in your ribs, so theres not much movement to be had, pivoting the body doesnt help much, as the ball isnt around the hip area so your point of pivot is not aligned to the ball. You'll find the trajectory of the ball is taking your hands up and away from the body slightly. Naturally, you wont be able to keep your hands pointing downwards or horizontal, because unless your stretch armstrong you'll need to bring the hands back towards the body at some point, which means changing the balls trajectory. Ok your gonna hold it most of the time, but making a ball with pace change path whilst its still moving brings all sorts of forces into the equation. Now anyone with Physics or Maths qualifications can tell you, an object travelling at a speed in one direction needs a force in the opposite direction to make it change path, so to bring the ball back to your body will need you to physically pull it in, rather than just letting it settle into a cup shape as you would taking it on the down.
To put it bluntly, taking the catch on a down trajectory means you can use soft hands to catch, almost letting it land in the hands rather than grabbing it, whereas on the up you'll have to be quite forceful to hold onto it and bring it under control. Like i say, probably can still hold 90-95% of them (if you've not snapped your shoulder) but why bring the risk into it. Theres nothing to be gained from catching the ball a few microseconds earlier.