As posted by Tac Foley (and I think the same method as mentioned by Steinar), it is easy to retract the receiver enough to rotate the takedown lever. Remove the magazine, and with the safety off, grip the frame and toggle knobs as shown in the following photo provided by Tac. Just squeeze so that the barrel/receiver group slides back until the toggle knobs contact the frame ramps and then rotate the takedown lever with your other hand. It works just as well for toggle-lock and grip safety Lugers.
