Ashi-gatame is an armlock achieved by tori doing a sankaku on one of uke's arms. This is seldom seen in judo but it does happen. Normally, if you try to sankaku uke's arm, he will pull the arm in and keep it bent to avoid ashi-gatame. This gives you an opportunity to do the Huizinga Roll. You will understand when you watch this video.
Ashi-Gatame & Huizinga Roll