Think of the receptors as keyholes and the molecules as keys.

The “correct” side is the right sided keys (D). The Meth-Key is larger and fits the hole nicely. The normal Amphetamine key is a bit smaller. This way it fits less well, but it is easier to fit the “wrong sided” key (L) because it is also smaller.

Now obviously what happens on the molecular level is different, but maybe this helps illuminate, why for one stereo-isomer the one substance is more potent and for the other stereo-isomer it is the other way round.