Tong Shen, also referred to as Penetrating Kidney, is a point on the Spleen channel that is used in conjunction with Tong Wei (88.10) and Tong Bei (88.11).
To locate Tong Shen find the medial superior margin of the patella. Note that this point isn't on the bone and it isn't in the muscle.
eLotus has an excellent video on how to locate and needle this point.
So you might wonder: why is a Kidney point hanging out on the Spleen channel? Shen Guan (also on the Spleen channel) also has a strong effect on the Kidney. What's that about? The simplest answer is that this is about a five element connection: earth (Spleen) containing the energy of water (Kidney) so that it does not leak away. That's why Tong Shen, Tong Wei, and Tong Bei have such a strong effect on the Kidney and why it is so useful for things like nephritis, impotence, spermatorrhea, and so forth.