Interestingly, Proverbs 3:3 also demands a sensory connection between the physical body and the spiritual virtues of "chesed v'emet". "Bind them around your neck, write them on the tablet of your heart" seems to have a concretizing purpose similar to swearing with your hand under your interlocutor's thigh: it connects your abstract moral intention with the reality of the body that you need to make that intention real.
Interestingly, Proverbs 3:3 also demands a sensory connection between the physical body and the spiritual virtues of "chesed v'emet". "Bind them around your neck, write them on the tablet of your heart" seems to have a concretizing purpose similar to swearing with your hand under your interlocutor's thigh: it connects your abstract moral intention with the reality of the body that you need to make that intention real.