I hear that phrase often both as a self dialogue and as a tip from a friend.
All it is is a cop out for not having good balance and/or rhythm.
I agree that if you swing hard and your balance is bad, you will hit the ball all over creation. However, no amount of swinging easy is going to make up for bad balance, regardless of the perceived improvement.
I will illustrate how swinging easy looks like it helps. If you are aiming at the middle of the fairway and hit a ball 30 yards off line and 250 yards…that ball is in the trees or a canyon. If you swing easy with the same poor balance and hit the ball 30 yards off line and 220 yards, you are in play.
This seems like it helps, but why mask the real problem. It’s like banging your head against the wall to make you forget about how much your broken toe hurts. Why not just relax and take some Advil.
Plus, swinging harder on balance creates more centrifugal force and centrifugal force helps you get better arm extension, which helps you square the club better.
As long as you have good balance and let the club set at the top by having good rhythm in the transition, you can swing as hard as you want.