Footlock, straight ankle lock, straight footlock, Achilles lock, or ashi-hishigi in judo is the position where the ankle is wrapped by arms and foot is under the armpit. It’s legal by IBJJF because there is no turning your foot in any direction and it’s also not affecting your knee.
In this video, we have a fantastic footlock that is still legal at the white belt level in the IBJJF competition.
Dean Lister’s Most Lethal Legal Straight Ankle Lock Explained