An automatic tranny in gear is "in gear". If the wheels aren't turning then nothing in the trans is turning.
The fluid part of the power flow comes from the torque convertor. The torque convertor has a pair of turbines (fans) inside. One is turned by the motor and the other turns the transmission input shaft. The coupling is similar to placing one electric fan in front of another and spinning the second one off the air of the first. The torque convertor is of course working with a much more solid fluid than air and is helped by the action of a stator mounted on a one way clutch, but the power flow becomes rigid at some speed (stall speed). When this occurs the connection is almost totally solid. This occurs at over 1500rpms on MBs.
On drag racing automatics they remove some of the fins from the driven fan to move the stall speed to over 4000rpm. The point is to get the rpms up for power before the car leaves the line. I only mention this to add understanding of the coupling effort.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician