It's from the"Silicon Sorcery" article in issue #292.
Prereqs: Dex 13, Dodge, Skill Focus (Tumble), BAB +8, 1+ ranks in Tumble.
Benefit: "You can attempt to cancel any critical hit made against you. Note the modified attack roll that confirmed that a critical hit has been scored against you, and make a Tumble skill check. If your Tumble skill check equals or exceeds the confirming roll (with modifications) then the critical hit is instead treated as a normal hit."
Basically, it makes the confirmation roll an opposed roll vs. your Tumble (with a minimum result of your AC), rather than a simple roll vs. your AC.
J