Z-guard is the variation of the half guard knee shield with the difference that the knee shield in z guard is much lower than in the traditional knee shield.
The knee frame is located on the opponent’s hip instead of the shoulder/armpit area, thus preventing the lateral movement of the opponent.
Of of the best and most famous z-guard players today is Craig Jones and you can see him use z-guard to set up sweeps, submissions and various leg attacks in almost every match.