The knees represent flexibility. Rigidity or locked knees are often linked to an inflexible viewpoint. Remember that the right side is generally masculine, and the left is generally feminine? I say this not as a dogmatic rule, but as a correlation that often holds true while I am exploring with clients.
Softening the knees is a learned art. It takes some time and effort for the body to give up it's need to know. 'I am at peace with the unknown' as an affirmation I sometimes use while moving the knees around physically, reminding them how to unlock. This reminds the mind that everything expressing through the body is expressing physically as well as metaphorically.
One of the purposes of the body is to conduct the life force. Locked knees block the life force in a few places, most notably the knees, hips, and lower back. To soften the knees, I usually begin with bringing the feet parallel to one another. Usually they are turned out, and rarely, in. When they are parallel, the hip sockets and the muscles through the pelvis have more space to let go. From that place, the knees can soften.
Often, when someone finally gets a sense of what softness means in the knees, this is accompanied by some sort of emotional catharsis. Again, it takes a while to be in a place where the knees are soft, so there are often a few sessions of mechanically going through the routine, as the body stubbornly resists. And then, eventually, the softness comes, and tears often flow.
We know this... letting go of the physical involves letting go of the emotional. When one part moves, every part moves. Once you find your feet, soften your knees. Become a crusader for truth. Make it a habit to question and relentlessly tear down your tired belief systems. Surrender to life as it is.
Oh, and as you kneel to the flood of the beauty all around, touch a plant... any plant, and watch as the lowliest weeds unfold as rarest flowers.