What is a lockdown in BJJ?

The lockdown is a half guard control where you triangle your legs around your opponent's trapped leg and hook their foot with your outside foot. It completely immobilizes their leg and prevents them from passing or posturing. From lockdown, you can sweep with techniques like electric chair and old school, or use it to set up other attacks. The lockdown is a cornerstone of the 10th Planet system but is effective in any no-gi game.