The present invention relates to a tool for selectively connecting or disconnecting components of a downhole workstring comprising a drive shaft (3) and a housing couplable to said drive shaft, and housing comprising first and second housing parts (21, 22) releasably connected to one another, transmission means such as a reduction gear mechanism (52-54) coupled to the drive shaft, the transmission means being arranged to selectively connect or disconnect the housing parts by rotating the drive shaft.