An underhook is established by looping your arm under the opponent’s armpit, as seen below, the wrestler in the white has an underhook with his right hand, and wrist control with his left:
Techniques with One Underhook
Establish a ‘high underhook’ by keeping your elbow high.
You can look drop your level and attack the leg closest, and/or snap-down with the underhook so that you can catch the opponent in a headlock or choke.
You can also step across the opponent and look for a hip throw or tai otoshi
If your opponent has wrist control and you don’t, you can lift your wrist up as high as possible and duck under his arm and take his back or grab a single leg.
Techniques with Double Underhooks
Tip with double underhooks, step your foot in between his legs, to prevent him attempting a wizzer or head and arm throw.