When you are passing someone's guard, one of the main concepts you need to understand is that the power comes from your hips. If you bend down with ab broken posture, you become an easy target for a sweep.
When you posture up, make a base and drive forward with your hips, you start putting pressure and this creates an opportunity for the leg drag.
In the video Sensei Douglas Santos demonstrates this with a lot of detail. When you land your knee on the mat, grab your opponent's collar to secure control.
From there it's easy to maintain control, using your free hand to initiate a choke, put pressure on their legs, and step back to complete the pass.
If you want to find out more about training at DS Team in Auckland, New Zealand, visit https://www.dsteamjiujitsu.co.nz