Isometric stretching assumes a position and pits one muscle group against another. It requires the tensing of the muscles when that position is reached and a balance is achieved. The position is usually at the very edge of the comfort zone, so when you do the splits, for instance, go as wide as you can. Center your body above your hips so that your body's weight acts as a force and counter it by applying pressure with your feet, tensing not just your quads but also your adductors to maintain a perfectly balanced position. There are many benefits to isometric stretching which include the activation of the pathways through which the muscles achieve greater growth. As with most stretched you should not use it before you do any other workout as isometric stretching contributes to muscle instability immediately afterwards.
Extra Credit: 30 seconds rest between sets.