Intuitively, most people sense that water "pulled" over a rudder will cause a
rudder to change direction of a boat in much the same way as water "pushed"
over a rudder does. However, intuition misses some things along the way.

First, let's take a boat sitting in the water, not moving the prop not turning.
The water pressure on each side of the rudder is the same, so turning the
rudder one way or the other does not cause the boat to turn at all.

Now, let's put the transmission in forward and turn the prop. The prop pushes
water aft. With the rudder centered, the water moving backward passes by the
rudder with the pressure the same on each side. If we turn the rudder to port,
the water being pushed back by the prop strides the port side of the rudder
(and NOT the starboard side) and the boat moves starboard.

Why?

Because the impact (pressure) of the water (molecules) on the port side of the
rudder was greater than the impact (pressure) on the starboard side. What
happened was that the water flowing past the rudder was *diverted* from its
path and the energy in the water was used to *divert* the rudder the other
direction. Remember the law of physics, "For each and every action there is an
equal and opposite reaction". The water went to port, rudder went to
starboard.

Absolutely neccessary for the rudder to force the back of the boat to starboard
is that the rudder forced water (from the prop stream) to port. "Equal and
opposite"

Now, let's take the same boat sitting in still water and put the transmission
in reverse and turn the prop. What happens? Well, the prop pushes water
forward. Where does it get "new" water from? Aft.

Now, here is the part where intuition comes apart. so, let's going slowly.

the water fills into the prop from aft because it is under pressure (i.e. water
pressure, or "water runs down hill"). the closer to the prop, the faster the
water fills. YET -- and here is the big part -- at all points aft and the same
distance from the prop have the same pressure pushing water towards the
spinning prop. THAT means that the pressure on one side of the rudder **is the
same** as the pressure on the other side. net, net, you can turn the rudder
any way you wish, but nothing happens because the pressure is the same on each
side, just as it is when the prop is not turning and the boat is not moving.

Still have a hard time with that? Well, let's look at it from another view.

The prop is in reverse and is drawing water into its circle and pushing that
water forward. Let's turn the rudder to port and see what happens as the water
streams by the rudder. Water hits the now aft side (former starboars side) of
the rudder? Kinda, but lets assume that it does. Which way is the water
stream deflected? Towards starboard? Then the rudder would push the boat (aft
end) to port. However, the water drawn over the rudder's port side hits that
side and is deflected towards port. Then the rudder would push the boat
(after end) to starboard. And equal and opposite reaction. Net, net, the boat
does not turn. The pressure on each side of the rudder is equal. Nada.

Net, net, you wanna steer with a rudder backing up, prop forward of the rudder,
you MUST be moving.