Glossary of Judo terminology
An "Sukasu" (Evasion) refers to an evasive maneuver (such as moving back, etc.) to avoid an Ashi waza (Foot / Leg techniques) being attempted by the opponent, causing the sweeping motion of the opponent's leg to miss. As the Uchi-mata-sukashi (Inner thigh reaping throw slip) Waza name implies, this is an evasive technique.
For example, when the opponent attempts to reap your foot from behind in an Osoto-gari (Large outer reap), you swing that leg forward to avoid his reaping motion. By thus missing his target, he becomes destabilized and easy to topple.