Get help to position yourself facing a wall with both legs extended up the wall. This is the starting position.
Allow the weight of the problem leg to help it slide down the wall.
When it has gone as far as it can, use the good leg to help it up the wall again.
The exercise is a passive flexion stretch, using the weight of the limb to gently force the knee into flexion. It may be easier to do the exercise with a helper and perhaps lie adjacent to a door opening, so that it is easier to swing the knees up.
The effect of tibio-femoral traction mobilization on passive knee flexion motion impairment and pain: a case series Maher S, Creighton D, Kondratek M, Krauss J and Qu X. J Man Manip Ther. 2010 Mar; 18(1): 29–36.