Bitting evasion - not a pretty picture - uncomfortable and damaging for the horse, negative influence for training. When you start to feel or see a change or reluctance to the bit don't persist beyond reason. You are communicating in the only way possible to demonstrate that he is no longer able to cope with or what you are asking. If he's opening his mouth, tilting his head or running through the bridle this should be enough to suggest that he may need another bit with a change of mouth piece or cheekpiece. Your bit choice should be specific to your unique partnership and should not be based on what your friends horse went well in - you have an individual style of riding and an individual choice of 'feel' which will ultimately determine which cheek piece is chosen and your horses facial conformation will require a certain shape, diameter size and action. When the correct bit is selected you will instinctively feel the difference, your horse will react in a positive and willing manor, with a greater level of relaxation feeling more responsive.
These pictures show disc like sores and cuts which can be caused by closing the horses mouth with restrictive nose bands forcing the inner cheeks between or onto the teeth. It is often the case that forcibly clamping the mouth over what is essentially a bit evasion or problem only ricochets other problems throughout the body - stiffness and tension through the neck and shoulders - impaired hind leg engagement - hollowing the back - inconsistency in the contact
Pinching the tongue leads to the horse opening the mouth and the arms of the bit squashing the lips onto the teeth