When the horse does something you see a wrong or negative, he's trying to tell you something. Putting a noseband on him to clamp his mouth shut is akin to taping a kids mouth shut when they're trying to tell you their shoes are too tight for their feet or they have a hurt somewhere.
Resisting the bit is anything other than holding the bit quietly awaiting your cue. If he's chomping on it, pulling on your hands - anything that's not right, he's trying to tell you something. Either the bit is not right for him or it is not being used correctly.
When a horse chomps, often a bit is hung too low, too high, it's too narrow or it's too wide. If the fit and size are correct, then he's not comfortable with how it's being used. Often I see people keeping pressure on the bit all the time - when pressure is applied to the bit, you should be asking the horse to do something. If he's doing what you wish, there should be no pressure on the bit. The release of the pressure is what rewards the horse for accurately responding to your request. So when you break down what you're asking into small parts, there should be releases along the way - let's say you're going to ask the horse to go straight, turn a 90 degree turn to the right then go straight again. You start off directing his nose straight forward and keep only enough pressure on to allow minute movements of your wrists for a change in cue. The pressure should not be enough to cause the pieces on the mouthpiece to collapse backward or gag the horse at all when he's moving forward straight in the direction you wish. Your legs play a large part too. ensure you're keeping the horse between your legs going forward straight via pressure from the legs. When you prepare to approach the turn, slightly tipping your right write to apply pressure to the right side while maintaining the supporting rein on the left and pressing on teh left side with your calf should cause him to turn. Maintain the pressure as he goes thru the arc of the turn, then release the pressure on the right side back to the same position and pressure as when you were going straight correctly before. Use only slight pressure to the right/left to put him on the track going straight you wish him to be on.
When the pressure is released and he's allowed to hold the bit quietly where he likes it when he's doing what you wish, that's his reward. Each tiny release of pressure and change to another type of pressure in between movements is a reward for the last few steps being done right.
Make your goal to be to go on the loosest rein possible while still maintaining control and his chomping diminish.
If you're using a standard 2-piece mouthpiece, try changing to a 3 piece like a french link or something. My older mare was started in a standard D-ring 2 pc straight mouth piece. She just wasn't responding well so I changed to a Myler D Ring Comfort Snaffle. The difference was night to day. She got quiet and began reacting accurately. The bit does the same thing as a standard snaffle but it is not possible to impart as tight a pinch and gag as a standard one. When I started the younger mare, I started with a sweet iron bit - same bit but with sweet iron. She'd chomp and mess with it and not pay attention. I switched her to the exact same bit her sister uses and she's quieted right down. I love the Myler comfort snaffles.
