The official GemStone IV encyclopedia.
Prop Up is a passive shield specialization that allows you to use your shield to avoid being knocked down. Each rank gives you a 1/3 chance to avoid knockdowns caused by criticals. In addition, each rank gives you +5 to defend against Sweep, Tackle, Hamstring, and Bull Rush.
Example
Dozens of long thorns suddenly grow out from the ground underneath your feet! One of the thorns jabs into you!
... hits for 3 points of damage!
... 2 points of damage!
Strike pierces thigh!
You instinctively drive your illthorn greatshield into the ground and prop yourself up!