There are many possible ways that this could be achieved; this tech includes two methods:
1) performing a well-aligned horizontal shinespark; starting 1 or 2 pixels from the door is an example position that works;
2) using an unpause where the fade-in ends exactly on the frame when the transition would trigger, delaying the transition by a frame.
For the shinespark version, about one third of positions will work, with working positions occuring in periodic windows that are each approximately two pixels wide, separated by bad windows that are approximately 4 pixels wide.
The unpause version refers to a buffered technique of standing at a specific distance from the door, pausing, and then unpausing while holding forward and run.
When starting facing the door, the position is approximately a 3-pixel window, at a distance of a 6-tile closed-end runway, with possible variations due to slopes.
When starting facing away from the door (doing a turnaround while unpausing), the position is approximately a 2-pixel window, at a distance of about 3.7 tiles.
See also: Tech Page