To offer the more complete story, here is the entire passage pertaining to this subject, from Cessna, Wings for the World, by Thompson, page 41:
With the advent of large slotted flaps in the C-170, C-180, and C-172 we encountered a nose down pitch in forward slips with the wing flaps deflected. In some cases, it was severe enough to lift the pilot against his seat belt if he was slow in checking the motion. For this reason a caution note was placed in most of the owner's manuals under "Landings" reading "Slips should be avoided with flap settings greater than 30 degrees due to a downward pitch encountered under certain combinations of airspeed, sideslip angle, and center of gravity loadings. Since wing low drift correction in crosswind landings is normally performed with a minimum flap setting (for better rudder control) this limitation did not apply to that manoeuvre. The cause of the pitching motion is the translation of a strong wing downwash over the tail in straight flight to a lessened downwash angle over part of the horizontal tail caused by the influence of a relative “upwash increment” from the up turned aileron in slipping flight. Although not stated in the owner’s manuals, we privately encouraged flight instructors to explore these effects at high altitude, and to pass on the information to their students. This phenomenon was elusive and sometimes hard to duplicate, but it was thought that a pilot should be aware of its existence and know how to counteract it if it occurs close to the ground.
When the larger dorsal fin was adopted in the 1972 C-172L, this sideslip pitch phenomenon was eliminated, but the cautionary placard was retained. In the higher powered C-172P and C-R172 the placard was applicable to a mild pitch pumping motion resulting from flap outboard-end vortex impingement on the horizontal tail at some combinations of sideslip angle , power and airspeed.