To defend against any attack, if you are able to respond earlier, the attacker cannot attack you successfully. For example, when you remove the attacker's hands as soon as the attacker is going to grab your shoulder, you are protected. The attacker cannot grab your shoulder in the first place.
How about if the attacker grabs and locks your shoulder?
If the attacker has already grabbed and locked your shoulder, that means, you could not do something beforehand, otherwise you would not end up in this situation. Let's watch this video. The attacker is putting pressure on your shoulder. So if you try to get up straight and release your hand, that will be difficult.
When you cannot make your move in one direction, you need to change your direction. You are close to the attacker's body in this situation. You can strike the attacker. Or, you can also hit the groin. Then grab the attacker's leg and pull.
The attacker might lose his balance. As a result, he cannot grab your shoulder anymore. Here the most important thing is, make your move suddenly without showing any obvious indications. Do not let the attacker know what you are about to do.
The attacker will find it shocking when he is off-balance and falls on the ground. Then you can strike the attacker while he is on the ground. Don't get to the ground with the attacker. You might need to fight with other attackers and get out of there.
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