X guard was popularised by Marcelo Garcia, although single leg X can be found in old Judo texts. The main advantage ofguard is that it allows you to spread your opponents knees very far apart and gives you great control over their base. The first and most important technique from here is the technical stand up sweep.
The next option most people will look for is the back take. This works better in the Gi but can work without if you are crafty and grab the hips instead of the belt.
