I played with a guy tonight and have gotten real settled in with my racquets strung at 40#s. I can really whip the ball and put heavy spin on it. He was a 4.5 and is probably a 3.5-4 now, getting his game back and he was commenting on how much spin was kicking at him. I noticed that all 3 balls had string marks on them within 15 minutes. I have done this before, but never thought too much about it. What is it a sign of? Just heavy string bite from the low tension?