Koppas, ask your coach if he can elaborate.
Anyway, I think that a common mistake is when a fighter attempts to extend their reach by lunging like a fencer. It's a big risk. When your weight is shifted forward, and your front knee is sharply bent, it's hard for you to recover your stance quickly, let alone mount a proper defense in the nick of time. Even if you're doing it to a lesser degree, you're still compromising balance and movement. Moreover, you'd also be absorbing more force when you're leaning into the shots.
The key point should bend your knees, just not so much that you're leaning forward like what Herb was getting at. You can't make your arms longer.
Also, you're knees must bend more when you're changing levels, i.e. going to the body. For If you don't bend your knees when you jab to the body, for 1, you're open, and 2, your punch won't be as solid. As a good rule of thumb, your punches should be on level with your shoulder.
So, try to ask your trainer what they mean by that. Either way, you'll get to bottom of it. You'll know what there reasoning is (or lack there of), and be able to make up your own mind.
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