both M1 Abrams tank and T-80 tanks have free shaft gas turbine engines and both of their engines have variable stator vanes in their power turbine part. It says in the picture below that these variable stators exist to prevent engine damage during gearshifts. How could variable stators prevent engine damage during shifts? By the way, PCA talked about in the picture is the automatic fuel injection system that regulates fuel injection during gear changes.
There is no clutch mechanism in the traditional sense in the transmission of the T-80 tank. Its role is played by the above-mentioned PCA. When changing gears, the PCA pedal is moved to a stop. In this case, the nozzle blades rotate 70-80° from the position corresponding to the maximum tractive power of the turbine, which actually leads to its stop (zero power position). Moreover, it is possible to move the voter lever only when the PCA pedal is depressed, since in the initial position it mechanically and electrically blocks the hydraulic servo gearshift. Thus, the guaranteed protection of the turbine rotor from misuse is ensured. By the way, the absence of such a mechanism in the GTE-ZT engine of the object 167-T led to the destruction of the turbine blades during the tests.
Also a picture of the AGT-1500 from M1 Abrams where it also shows the variable power turbine stator blades: