propellers 01

The repulsion between the rings prevents the molecule from being arranged in the plane. So one of the rings bends towards us and the other at the bottom. In one enantiomer the ring on the right is bent towards us and in the other to the bottom.

propellers 02

To distinguish both enantiomers, a turn is made from the ring that goes to the bottom to the one that comes towards us. If this turn is clockwise, the enantiomer is M, if the turn is in the opposite direction, the enantiomer is P.