Prof. Vin demonstrated the X-Pass in class recently. As you can see in the video below, break any grip your opponent may have on your far ankle before stepping your near leg up by your opponent’s hip to stuff his near leg.
Then, gripping his lapel and the outside of his near knee, high-kick your far leg back and into the knee on belly position.
Staying tight in the transition from knee on belly into side control is key to avoid giving your opponent the opportunity to escape.
In the video below, Prof. Vin shows how to maintain heavy pressure by shifting the angle of your knee from the center of your opponent’s chest towards his/her armpit while keeping constant hip-to-hip contact and your knees off the mat as you settle into side control.