GURPS Martial Arts pg 77
page 5 of http://www.sjgames.com/ill/img/retailerimages/Munchkin%20OTD/Game%20Support/GURPS/GURPS_Martial_Arts_Techniques_Cheat_Sheet.pdf mentions too
Kromm in 2018 http://forums.sjgames.com/showpost.php?p=2216731&postcount=11
- On parries with legs or feet: GURPS Martial Arts, p. 123 makes clear that only Brawling or Karate works, and only vs. "low-line attacks," defined as attacks on the feet, legs, or groin. Use the usual Parry score.
On parries against attacks with legs or feet: Any parry can try this, but Boxing (per p. B182) and Sumo Wrestling (per p. B223) are at -2. The Low-Line Defense technique (Martial Arts, p. 77) can buy off this -2. Yes, "low-line" is used differently here, but note that it qualifies "defense" and not "attack."