The shudder occurs when you're running the engine in too low revs to actually maintain the speed. Example: going uphill at 15 mph in 3rd. What you should do is drop a gear and accelerate. But if you stay in gear and try to cure it with more throttle, there will be said shudder from the engine, because you force it to do something it can't do.
It's not exactly good for the engine, I don't know what bad it does though. I'm pretty certain one of the automotive geeks of this forum can explain it in detail.