If you're in turtle position and your uke/partner reaches over your back, this is an easy way to catch them off guard. Make sure to keep that arm trapped in your armpit and they'll never make that mistake again.
Steps for the Arm Roll:
- partner reaches over the top of your back
- windmill arm to capture partner's arm and grab your far lapel
- roll over shoulder to secure Ushiro Kesa Gatame
- free hand is grabbing partner's far pant leg
- keep scooting butt into partner's armpit to prevent them from sitting up
- transition to Yoko Shiho Gatame if they attempt an outside turn escape