mechanically the Fwd thrust lever & reverse thrust lever are interlocked mechanically that only one can be moved at a time.
If the fwd thrust lever is fwd the r/T lever will not operate & vice versa.
On the detent part.....Initial R/T lever mvmt is till a particular point through interlocks that prevent further R/T lever mvmt until the T/R sleeves are deployed.Thereafter there are Two detents on the lever itself which provide a mechanical feel to the location when reached.
regds
MEL