It's All About The Heart
According to HeartMath Institute:
"....the heart is the most powerful generator of electromagnetic energy in the human body, producing the largest rhythmic electromagnetic field of any of the body's organs. The heart's electric field is about 60 times greater in amplitude than the electrical activity generated by the brain. This field, measured in the form of electrocardiogram (ECG) can be detected anywhere on the surface of the body. Furthermore, the magneticfield produced by the heart is 5,000 times greater in strength than the field generated by the brain and can be detected a number of feet away from the body in all directions."
The electromagnetic field of the horse's heart is 5 times greater than ours, making their sensitivity to energies around them more acute, thus picking up the energy we are putting out. For example, our emotions carry different energies. The horses will feel these energies and respond to them showing us what we are feeling, even when we are unaware ourself.
Interestingly the horse's heart beats three times slower than ours. When our heart and body connects to this energy we feel relief, a sense of well-being and our bodies begin to heal inviting us to slow down and experience what's going on inside of us.