A well placed underhook is a window to your opponent’s back because ideally there are no limbs to block your body from moving there. That doesn’t mean it’s an easy path (or even one that you consistently hit on your best training partners), but it is a strong threat.
First, establish the underhook, attach your hand to your opponent, and create a strong forward off-balancing movement. As soon as you have shucked your opponent forward off-balance, take that opportunity to come out to the back corner of your opponent and draw your inside knee out enough where you can use it as a stable base but not so much that the hook comes out. You should be sliding into a back position with one hook already in. This is made possible by the strong hand connection to your opponent’s back which we discussed earlier. The attached hand makes the off-balance possible which makes the back take possible.