There are times when you can not get the underhook. In those instances it is very important that you have other options to threaten your opponent. In this video I show a great sweep/reversal (depending on if you use your legs to elevate them) that starts from the Half Guard position with a Kimura grip.
Obviously if I have the choice I would prefer the underhook, with that being said threatening your opponent with a submission can be a great way to transition from bottom to top.
When entering for the Kimura grip I suggest framing on the bicep and the face (reference video) to insure that you dont get crushed. One of the big problems that people face with obtaining the Kimura grip is the spacing to actually set up the grip. I highly recommend stretching your back to get your hand to your opponents wrist as opposed to trying to move your opponents arm. This little detail can be the difference between success and failure, especially for the smaller grappler.
Once you have that hand on the wrist I like to straighten my top arm to get the second hand around the elbow. This is a little different than the way most people do it and I suggest really focusing on the video to see how my entry differs from other peoples.
Once you have control of that grip you can bail on Half Guard. This is the most difficult part of the move as it takes a lot of faith in your technique to abandon a neutral position. But it is very important that your legs are not tied up in your opponents because it will prevent you from burying your head under your opponent.
The NUMBER 1 reason that people fail at this sweep is because they do not bury their head underneath their opponent. Please reference the video to see how deep I go under my opponent before going to my back.
Once you are flat on your back it is important to push with that elbow on your opponents hip. This not only gives you the ability to push your opponent over (adds power, but is not the main powering force) it removes the danger of your opponent transitioning into an armbar.
Lastly, never let go of the Kimura grip.