One nice thing about this submission setup from butterfly guard in particular is that you can hit it on either arm. If you're underhooking and your partner isn't interested in giving you much space to get that arm, or (more likely) they post when you go for the classic sweep, they're giving you everything you need for this second variation.

As they post to prevent the sweep, reach for their far arm with a palm-to-palm grip, taking care to make sure that you are just above their elbow. Now use their elbow to start pulling you over to that side of their body (you are going to go from hip to hip here, so you might need to scoot along the ground with a shrimp movement, or you might be able to pull yourself with your partner's arm if they're much bigger than you. Once on this far side, the submission is the same, but your setup might be even easier, since you are making the proper grip to get the finish much earlier in the process, before you even scoot all the way over to the other side.