I am assuming you are talking about looping against topspin.
If you are closer to the table, you will probably be looping at the top of the bounce or even before that (top of the bounce is ideal). Because of that you will have to raise your backswing and spin the ball forwards to prevent the ball from going out. This requires your forearm to snap and your paddle angle to close a little. If you want to hit the ball harder, feel free to rotate your body to get a bigger backswing/swing.
ATTENTION:
If you are close to the table and you are looping somebody's serve, then be a little careful. Serves are generally much lower than the other shots in the rest of the point, so you will have to spin the ball up a little bit for those, but not too much to loop them out. Also, you could get different spins, obviously.
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If you are farther from the table looping, you are doing one of two things: looping somebody's block, or counterlooping. Since you asked the question about forehand topspin, you are probably not counterlooping.
Looping somebody's block farther away from the table means either top of the bounce or after the top of the bounce.
Either way, you will need to spin the ball upwards a decent amount, but also give the ball enough forward momentum to send the ball past the net. So swing forward and spin upwards. You will need to use your forearm to spin upwards, but in general, your movement should be forwards.
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Using body and legs for looping in general:
Body: Turn your body for the backswing and the swing. It helps generate consistency.
Legs: Since looping generally requires upward motion, a lot of people get lower on their backswing, and then back up a little on their swing. However, you don't really have to force your legs to do this. If you practice, this will come naturally.
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There is an older thread about the forehand topspin that you should look at if you have other questions.
https://www.tabletennisdaily.co.uk/...13998-Question-on-Biomechanics-of-the-FH-Loop