1) create a room with a southern passage.
2) Push and hold Down and go through the passage, do not let go of Down.
3) Also push Left and note that when you reach a solid object you won't move left, you keep walking into the solid object.
4) Let go of Down then press Down again as well as Left, it will now work correctly and once you encounter a solid object you will move left.
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In summary, southern doors mess with the down+left direction combination. Tested with diagonal movement.