Some times called the straight ankle lock, or the Achilles hold, this ankle submission is a bugger. A guy can stack you in the guard, pop up and grab a leg as soon as your ankles become uncrossed. When he does, he basically puts a guillotine choke on your ankle and finishes the submission. It can happen really fast.

Just like this vid shows.

If you want to learn more on how to apply this ankle lock, look over at this instruction by Steven Kesting.

The real video gem is watching Carlos Machado go into the details of escaping and blocking the submission. The vid below is 8 minutes and goes into a lot of specifics.