The phrase "opposite knee" means the knee that is on the other side of your body, not the one you are currently referring to or looking at. Full definition
Step one foot out to the side, knee bent, toes facing out and heel in line with opposite knee. (dailyburn.com)
Alternate crossing your ankle over opposite knee to achieve a deep stretch on both sides of the body; then switch sides. (health.com)
With your hands behind your head, try to touch one elbow to opposite knee in a bicycle - pedaling motion similar to the illustration above. (eatrunanddone.com)