The weeks after a knee replacement are a window: this is when the new knee most easily gains the bending and straightening it will keep. Daily exercise is how you use that window.
Little and often works better than heroic efforts. Two short, honest sessions a day - the way the program schedules them - achieve more than one exhausting session followed by two days of recovery.
Report your pain truthfully after every set. The program uses your reports to progress you, hold, or step back. It is built to adapt to a bad day - skipping the session entirely teaches it nothing.