When dry firing your MPR you need to unscrew and release the air from the chamber and leave the cylinder loose.
The MPR air cylinder is then unstable and when moving the rifle the balance can feel different to normal.
Using the Dry Fire adaptor means you release the air from the chamber by unscrewing the cylinder enough to insert the adaptor, check that the barrel is clear and then insert the adaptor. Once the adaptor is inserted tighten the cylinder. At this point the cylinder is tight but the air is not released to the rifle chamber allowing dry fire but also the cylinder no longer moves.
Note: Colour May Vary.