Mr. S. Keene correctly wrote :

Car A is shoved onto spur / engine backs onto b and couples with A with B / Cars are reversed on the main / B is placed onto spur and A is cut off on Y / locomotive proceeds to other leg of Y and onto spur, coupling onto B in spur places car.

Steve Keene

Brother Hewitt correctly wrote :

ENG=> A=> SPUR
ENG=> B=> A=> TOP WYE=> B=> SPUR
ENG=> A=> B's 1st POSITION
ENG=> B IN SPUR=> A's 1st POSITION
ENG=> TOP WYE

SIMPLE, NO?