Waki-Gatame is normally thought of as an illegal technique, and the version often seen in competition, where it is done from tachi-waza and is combined with a throwing technique, is indeed illegal. But here are two legal versions.

One is done from a groundwork situation, where uke is trying to do an Arm Roll. The other is done following a throw (Soto-Makikomi) rather than with a throw. The follow-through is what makes it legal.