PROM is an abbreviation for 'Passive Range-of-Motion'.
Using a helper
The passive range of motion needs a helper to take the knee through the available range of motion without the patient trying to help, so that the muscles are relaxed.
Patient Self-Assessed Passive Range of Motion of the Knee Cannot Replace Health Professional Measurements. Borgbjerg J, Madsen F and Odgaard A. J Knee Surg. 2017 Oct;30(8):829-834. doi: 10.1055/s-0037-1598174. Epub 2017 Mar 1.