Retention is gained by the flexible retentive tip of the clasp engaging an undercut of an abutment. Retentive undercuts exist only in relation to a fixed plane.
If an undercut is found on a diagnostic cast, and the cast is tilted in another direction, the undercut can be eliminated. Likewise, in the mouth, if the partial denture does not have a single path of insertion (as dictated by guiding planes) the prosthesis could be rotated so that the retentive
undercuts would be eliminated.
A "slippery slope" in the Partial Denture Department refers to the patient who only has teeth #22 - #27 left and needs a partial denture.
Dr. DiTolla gives insight on how to achieve retention and support in the scenario. Originally aired on Chairside Live episode 102