This stretch is sometimes called the runner's start because the position you are in resembles that of a sprinter at the starting block. It mainly stretches the psoas muscle located just above the top of the thigh.
Crouch down on the floor with both hands and knees on the ground. Put one leg forward with your foot on the floor so that your front leg is bent at the knee at about a 90 degree angle. Now extend your rear leg in back of you so that it is almost completely straight (with just an ever so slight bend) and so that the weight of your rear leg is on the ball of your rear foot with the foot in a forced arch position. Now we are in the position to stretch (notice that your rear leg should be in pretty much the same position that it would assume if you were performing a front split).
Keeping your back straight and in line with your rear thigh, exhale and slowly try to bring your chest down to the floor (you shouldn't need to bend much further than the line your front knee is on). You should feel the stretch primarily in the upper thigh of your rear leg but you should also feel some stretch in your front hamstring as well. Hold this position for at least 15 seconds. If you wish to also stretch your rear quadricep from this position, you can shift your weight back so that your rear leg makes a right angle with your knee pointing toward the floor (but don't let it touch the floor). Now, without bending your rear leg any further, try to force your rear knee straight down to the floor.
Now repeat the same stretch(es) with your other leg in front.
For an isometric stretch, you can do this same stretch in front of a wall and instead of putting your hands on the floor, put them in front of you against the wall and then push against the wall with the ball of your foot (without decreasing the "stretch" in your psoas).