The filmy band is actually a fold in the capsule of the knee, but that is not appreciated in this illustration.
This illustration gives a better example of how the plica folds in from the capsule surrounding the knee joint.
What causes a plica to become symptomatic?
If a plica is traumatised, it may become inflamed and eventually thickened, when it may be nipped between the bones of the knee causing sharp pain because the synovium itself is filled with pain-sensitive nerves.
Synovial Plica Syndrome of the Knee: A Commonly Overlooked Cause of Anterior Knee Pain. Lee et al. Surg J (N Y). 2017 Jan; 3(1): e9–e16.
Synovial Plicae in the Knee Boles CA and Martin DF. American Journal of Roentgenology. 2001;177: 221-227.