Some things that I have to still try to remember while on bottom:
- Keep elbow inside the knee of the hook, when going for basic hook sweep with an underhook and head on chest (forgot this again)
- Stay really close so that opponent can not pummel for underhook
It has been difficult to keep the elbow in mostly because it feel more natural to have it outside to get the proper angle for the sweep more naturally. The key to being able to keep to elbow inside is to be really close to your opponent. Also, if you are using the overhook variation with a belt grip, keeping the elbow in is most likely impossible.
Marcelo doesn't use the elbow as support while sweeping (see on the video), but as I recall, he has the elbows inside when hand fighting.