At the most basic level guarding occurs when your therapist pushes your knee one way and you push right back, negating his attempt to help you gain flexibility as you try to protect yourself. This is called Protective Muscle Guarding.
Using pre-habilitation as a strategy to reduce both short and long-term strength deficits following knee surgery has recently begun to receive the serious attention it deserves. We now know definitively that knee pre-hab enhances outcomes. Dr. Carl Freeman presents comparative data between pre-hab methods here.