Avoiding a guillotine choke is all about staying out of the range of the guillotine. This is done in three ways:
1. Keep your head back. Essentially look directly at your opponent, rather than dropping your head and shouldering through.
2. Shoot low. Aim to put your head around his hip level. Its biologically impossible to wrap up someone's head when their neck is at wrist level. In this situation, head position (up or down) is less important, but keeping it up should be a priority. The only reason not to keep it up, would be to protect the chin from a knee, which depends on the standup skills of the opponent, and his apparent willingness or aptitude to use knees.
3. Shoot to the center. In this case the goal is to keep the head centered along the opponent's centerline. This does not make it impossible to be guillotined as effectively as a lower shot, but it does make it easer to sense the set up as your opponent's core turns to align his arm with your head, as well as makes it easer for you to stand up and move for a body clinch, or to drop to a knee and get a lower double/single. When shooting to the center, keeping the head up is very important.
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I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit