Here's the rest of that line: ... you don't hit the ball solidly. Whatever speed you might gain by trying to go harder is wasted because you lose control and mis-hit the shot. You're probably not gaining any speed. Like I joke with some of my students: Your backswing is faster than your downswing.
If you want more distance, your focus should be finding a rhythm that allows you to produce speed without giving up good contact. Consider the swing thought Jack Nicklaus used on tee shots: Finish the backswing. You want to stay smooth and deliberate going back, let everything wind up behind the ball. Then you can come into impact from a strong position, not a weak position that's all hands and arms.
Think about when you hit a bunch of drivers in a row on the range. You get into a good groove. Why? First, you're not reacting to all the obstacles you see on the course. Second, you don't have mechanical thoughts—you're just swinging. That's what you need to do when you play.
DIAL IN YOUR DRIVER
What can you do when you practice to find a rhythm that creates speed and control? Tee three balls in a line (below), and hit them in succession, stepping up to the next ball as soon as you finish your swing. You'll instinctively know to stay in balance, so you won't overswing. Do this drill a few times, and try to really internalize the rhythm and flow of it. You'll hit more shots in the middle of the clubface and get the most out of the speed you're creating.