Receiving a soccer ball with your thigh is a great way to control the ball if the ball is coming in not high enough yet to receive it with your chest, for example, or if the ball is not low enough anymore to receive it with your foot. the thigh provides a very big surface again to receive the ball, to control the incoming ball. It helps you to really cushion the ball, slowing it down no matter what speed the ball's coming at.
Again, to get ready for the ball you need to keep moving on the toes. Make sure that you're behind the ball. Instead of reaching and angling the knee out to the side, you want to be behind the ball with the entire body so you at least stay in possession of the ball. The knee comes up slightly. You have like a 45-degree angle. The ball is coming in. You expect the ball in front of your body, and you go with knee slightly with the ball to absorb the energy of the ball and slow down the ball as it comes in. Another way to use the thigh is as well to have the incoming ball already move to one side or the other to avoid pressure situations and move into the open space that is available and get away from your defender. That's the basic idea of how to the ball with the thigh.