Stop Hit

Stop Hit was a maneuver/technique found on page 55 of GURPS Martial Arts in 3e. It worked a lot like the "simultaneous attack" of Palladium books: by surrendering active defences and attacking into an attack, your target cannot defend against you.
GURPS was more flexible than Palladium though: they allow your target to abandon their attack to use an active defence at a mere -1 to hit.
Whoever rolls to hit first (determined by a quick contest) if they hit and do damage, got to apply half the shock penalties on the loser, which would then apply to their roll. A stun wouldn't prevent an attack as usual though, only apply the greater of -4 or damage (not shock, so even High Pain Threshold guys would be compromised equally).
Ties meant true simultaneity with shock penalties applying to neither (next turn instead).
In 4e's Martial Arts it was changed into an attack option instead. Not to be confused with the similarly-named Stop Thrust option for Wait.
Peter V. Dell'Orto wrote about at https://web.archive.org/web/20170625110950/http://dungeonfantastic.blogspot.com/2013/05/gurps-melee-academy-stop-hit.html
GURPS Martial Arts 111 mentions a -1 to Margin of Success for swing attacks
Category:Melee Attack Options