This lasso guard sweep can be initiated when your opponent pushes the knee of your leg lasso through their armpit, eliminating the frame you had with your shin, and positioning them to be able to pin you with chest to chest control.
Once your opponent has pushed your knee through and begins flattening you out, straighten the leg then make a circular motion with your leg for momentum to roll yourself underneath your opponent. From here, if you still have control of their sleeve, you can roll them over for the sweep.
If you are able to grab a hold of the opponent’s foot or pants, or secure an underhook on their leg, this can be used to make the sweep stronger.