There are several variants of the M42 105mm, the one you have sounds like the Auto-Takumar 105 f/2.8. If so the little lever is (was) used to reset the aperture after taking a shot. The earlier Takumar variant was a preset lens with two aperture rings, you set one to the aperture you wanted, focused, and then turned the other aperture ring to close the aperture to take the shot. The Auto-Takumar allowed the camera to close the aperture just before taking a picture instead of you having to turn the ring manually, which allowed you to focus wide open and then take the shot. The little lever was then used to "re-cock" the lens for the next shot.
Of course this only worked on the cameras that were made to function with this lens. The mechanism will not work with new cameras as they have no linkage to actuate the lens. Later variants such as the Super-Takumar and the S-M-C Takumar did away with the lever and the lens was actuated by the camera without having to "re-cock" the lens. On today's camera these lenses must be used in manual (or Av) mode and the aperture ring turned manually.
Here is a link to the
Auto-Takumar 105mm, if this isn't the lens you have then post a picture or the exact description and I'm sure someone can help.