The Dutch drop was to get the car from the north end of the engine to the
south end (for example), but without a run-around. So, the moving car was
allowed to roll past the locomotive that has run away from the rolling car,
stopped, reversed, thrown the switch, and run into what was a trailing point
switch. The switch is then thrown again and the car rolls past, putting the
car at the other end of the locomotive. The locomotive now throws the
switch, runs out of the spur, catches the rolling boxcar (or the brakeman
has stopped it), and the train reassembled.

As for why - if there is switching to be done, and there is no way to get
the car to the other end of the train except to travel several miles to a
run around, well, many crews won't suffer along with spending literally
hours running to a run-around just to run back. Hence, the Dutch drop.

Tom