The tuning fork was invented in 1711 by British musician John Shore. The main reason for using the fork shape is that it produces a very pure tone, with most of the vibrational energy at the fundamental frequency, and little at the overtones (harmonics), as is the case with other resonators. So when the fork is struck, little of the energy goes into the overtone modes; they also die out correspondingly faster, leaving the fundamental tone. It is easier to tune other instruments with this pure tone. Just like an instrument, we can go ‘out of tune’ and the tuning fork reminds the energy body of its natural frequency.  

By setting the base of the tuning fork on your body, you act as a resonator that amplifies the sound of the fork. These sound vibrations travel through your physical body as well as your energy body. You can get tuning forks in an assortment of different frequencies. They have the C scale notes, meridians, planetary frequencies and many others. 

I have found that is it good to use only one fork at a time as some frequencies can cancel each other out, but experiment with them, there is a whole body of research still to be done!