Start off by imagining the position of your pelvis when you are standing up or sitting straight up in a chair. It will be facing forward and
perpendicular to the floor.
If you hang in that position you are hanging straight down or SD.
Now imagine you are lying down on the floor with your legs straight out ... or better still ... lying out flat and floating 3 feet above the floor. In that position your pelvis will be facing up to the roof and
parallel to the floor.
If you hang in that position your dick will literally be hanging between the cheeks or BTC.
Some guys sit in a chair and lean back, tilting their pelvis upwards and taking them away from SD and a little closer to BTC. Sometimes they'll elevate their feet to assist.
To achieve a true BTC I like to:
- sit right on the end of one weight bench and attach the hanger;
- lie back flat on the bench;
- and put my legs / feet up on another bench of exactly the same height.
In that way I am truly parallel to the floor and the weight is hanging freely between the cheeks ... a true BTC.
I guess the crucial part to think about is which way your pelvis is facing ... forward (SD), upward (BTC), or 45 degrees (really a blend of the two ... but most would also call this BTC).
I hope this helps.