Hey there! JayyFen here, I've been coaching RL full time for well over a year, and I recently analysed a C3 that was rotating backpost, but still doing this one mistake, so if you're champ you'd want to listen up.
When the player rotated backpost, he got into a perfectly good defensive position, and then immediately started creeping forward. His positioning and timing of his rotation weren't necessarily the issue, but it's what he decided to do after completing it. Just because you've reached the correct position doesn't mean you should immediately leave it.
What I mean by this is that if the opponent still has possession and the play hasn't moved yet, pushing forward immediately and unnecessarily can put you in awkward positions as you're essentially giving yourself less time to react to a potential shot, pass, or even a bump, all because you felt the need to move.
Instead, focus on being patient with your positioning, if you've rotated back post and the play hasn't moved, there's absolutely nothing wrong with holding your position until you have a reason to push forward, and once the ball moves and the play develops, you can adjust accordingly without putting yourself at an unnecessary disadvantage.
This is something I constantly emphasize whenever I coach players, as good positioning isn't just about knowing where to be, but also when to act accordingly. Lower ranked players overlook the fact that you don't have to be moving all the time, simply slowing down and waiting for a challenge to happen as last man is simply the best decision to avoid overcomitting.
So the next time you push up to backpost, keep an eye out on whether you creep up to front post or not before the play does.
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Attached find the video reference, looking forward to your thoughts:)
https://reddit.com/link/1xgaqn5/video/f86frnz2xvxh1/player