It is important to understand the difference between isometric and isotonic exercise. Using dumbbells for weight training is an example of isotonic exercise. There is joint movement and muscle contraction happening at the same time. Sometime the body is used as a resistance force.
In Isometric exercises the muscles may contract but there is not motion in affected joints. This is very useful for developing strengths of muscle groups. Yoga can be used as a part of any regular exercise regimen when done properly. Do not try complicated yoga poses without attention from yoga instructor.