• The push stick was flawed, in that it was too thin, and broke in half.
• When the push stick broke, his hand's own momentum carried his fingers downward and to the left, directly into the path of the blade.
• Without a rear table extension, the weight of the cut board acted like a teeter-totter, and lifted the front of the board. This applied excessive pressure on a weak push stick.
• As the board lifted, it struck the backside of the sawblade, and lifted with a jolt, upward and to the left. This was the impact which finally snapped the push stick.
• The saw blade height was too high. The saw blade should be about 1 to 1 1/2 inches above the top of the wood. More than this leaves too much of the blade exposed to your hands, but less than this will result in more frequent kick-backs.
• His rip fence was modified, and too tall. This forced his hands to remain on the left side of the fence during the rip, with little chance to move to the right.

General Principle
When using a push stick, your hand is behind the tail-end of the workpiece, and the pressure you are applying is generally forward, with only moderate downward pressure. In the event something were to go wrong, your hand has the tendency to go forward and down, which is heading toward the blade. While the push stick discussed above broke in half, the same could happen with a kick-back, where the stick would flip out from under you, and your hand would still continue forward and downward.

With what I call a push shoe, the device is more stable, and less prone to being knocked out from under you. The push shoe holds the workpiece down better not only at the back, but also prevents the front of the board from lifting up. If the front of the board comes up, you risk having an "Over-the-top-kick-back", when the back teeth of the blade catch the wood, and it rides over the blade. (See Today's Woodworker Magazine issue 53 p.28 for my discussion of various types of kick-back) The reason for the increased stability is because your hand is positioned over the tail of your workpiece, not behind it. Furthermore, your hand's pressure is applied directly downward, not at an angle through the length of the push stick. And finally, the sole of the push stick provides a longer contact area with the workpiece, which is what prevents the front of the board from lifting.

Construction
The push shoe in my shop is very simple. I cut it from a scrap piece of 5/8 inch melamine without even drawing a cutting line. It took about 30 seconds to make, and has been used for 3 years. It is getting time to replace it, as the heel has been cut back to about 1/4 inch. Mine does not have a handle, and I'm not sure I would want one. (My 4 fingers would then be on the left of the shoe-through the handle- instead of remaining on the right hand side.)

If you make a handle as shown below, do not use particle board, as the handle could break or collapse. Probably the best material to use is high density maple plywood (See Plywood Types for a discussion on high density plywoods). Regular plywood has too many voids, and the core-laminations are not as strong.

Size
Don't make the shoe too big. While many woodworkers want their hand the farthest from the blade they can, making it too big will make it less stable. Mine is only about 6 inches tall, but you can go a little taller. The sole is also about 6 inches, but that doesn't matter much. If you do put in a handle, then you will probably need to make it taller to keep your fingers out of the way of the blade.

The drawings below show the shoe with, and without a handle.