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When you hang back on the court you make it easier for your opponent to hit crosscourt directly into the back corners.
The problem you have is if you go too far up the court you will be in a good position to cut out the crosscourt but you will not have time to react and play a quality shot from it. So there is a balance to strike here.
So from this T position here, higher up the court I can stretch across and cut out any ball that is going crosscourt to the corner, and the same thing on the backhand.
If my position is too deep and I stretch across, you should be able to see the back corner. The same on the backhand. you should be able to squeeze past my racket into that back corner.
You can liken it to a goalkeeper coming out to meet an attacker in a one on one situation. If the back wall is the goal here, as I move forward the less of the wall (or goal) is available for you to see (and hit).