An assembly (127) for a wind turbine (100) includes a housing (126) having a first bearing (150) and a second bearing (154). A shaft (142) extends axially within the housing (126) and is supported by the first bearing (150) and the second bearing (154) for rotation relative to the housing (126). A radially outer portion of the shaft (142) includes at least one shaft engagement formation (146) positioned between the first bearing (150) and the second bearing (154). A retention mechanism (156) is moveable axially between: an engaged position in which it can engage the shaft engagement formation (146), such that rotation of the shaft (142) is constrained; and a disengaged position in which the retention mechanism (156) cannot engage the shaft engagement formation (146), thereby allowing rotation of the shaft (142). Additionally, a method of operating such an assembly.