This animation is of a simple double acting traction engine operation with no reversing to illustrate the valve and cylinder motion. (double acting because steam is admited at both ends of the cylinder to push the piston in each direction)

The stop valve opens to admit steam into the valve chest. Assuming the valve is cracked open .......if its not ....thats when the driver moved the flywheel by hand.....the steam flows to the cylinder and expands pushing it to one end.

As the cylinder moves towards the other end the valve fully opens and then closes. The flywheel takes the piston over top dead centre and the piston starts to travel back. The valve opens to admit steam to the other end of the cylinder. At the same time the valve is letting the expanded steam out through the exhaust which is the recess in the middle of the valve.

So, how do we make the engine go backwards?

Introducing the reversing link........Essentially what is done is to split the valve rod into two by the introduction of a curved link. The top and bottom of this link is connected to two eccentrics ....one eccentric to the top and one eccentric to the bottom.

The reversing link can be lifted up or dropped down (as shown) so that depending which rod is in line with the valve rod, depends which eccentric drives the valve rod.

The eccentrics are not at 90 degrees to the throw of the crank but in advance.......say about 15 degrees. This means that the valve is opening just before top dead centre of the piston.

By putting the two eccentrics such that each is in advance of the top dead centre of the direction of rotation the engine will run in reverse.