Out of an old Garrett TPE331 pilots brief and ops tips booklet:
6.1 Normal Ops procedures checklist
PROPELLER -ROTATE BY HAND
-A valuable practice toward developing a "feel" for characteristic engine sound and rotational resistance; helps to establish a baseline of both feel and noise so that, should a change be detected on future prop rotations, appropriate maintenance investigation can take place.
- If an abnormal resistance is noted, hand rotation should be stopped at the point where the resistance is most obvious; representing 180 degrees displacement of the main rotating group (neutralizing the thermally caused imbalance as cooling continues)
- Rotational freedom should be re-checked after about three minutes of additional cooling (depending on ambient variables).