Shirl:
The difference is that when the longitudinal axis of
the airplane is not lined up with the runway centerline
(forward slip), there is much more drag.

"Nils Rostedt" wrote:
I think this is true only in the no-crosswind case when the
relative wind has the same direction as the runway. If there
is crosswind, the relative wind direction is from a direction
more or less to the side, depending on the crosswind component.
Therefore, in a crosswind side-slip approach, even if the airplane's
longitudinal axis is aligned with the runway there is some degree
of additional drag as the airplane is not aligned into the relative
wind. (In comparison, in a crabbing approach the airplane is aligned
much closer to the relative wind.)

Agreed.

It's true that the action of the pilot is the same; however, in a
forward slip (for loss of altitude) it is cross-controlled to a greater
degree so that the airplane is actually coming down sideways (but in
line with the centerline) so as to expose more of it to the oncoming
wind. Yes, there's drag in a side slip (for crosswind correction) too,
but not as much, and the whole point of the side slip is to keep the
airplane lined up with the runway, not to lose altitude.

Yes, in a forward slip where you want maximum descent speed it's no problem
to use full opposite rudder. In a crosswind side-slip, the crosswind itself
does much of the job the opposite rudder does in a forward slip, so less
rudder is needed.

Agreed.

Did we reach a higher level of confusion? ;-)

No confusion here. ;-)
Again, I only meant to point out the difference and their uses because
in an emergency situation where losing altitude quickly and/or fighting
a strong crosswind were paramount to getting the plane down where you
have to, knowing the difference and using the correct one could mean the
difference between success and disaster.