There are thermal effect and non-thermal effect for the operation model of FIR.
Thermal effect is the phenomena when the cells of body surface receive energy from near infrared and generated heat that make us feel warm. It belongs to physical mechanism
Non-thermal effect is the phenomena when the cells of deep layer of body absorb FIR and activate cell to accelerate circulation and improve their operation function. It belongs to psychological mechanism.
In thermodynamics, thermal effect belongs to conduction, and non-thermal effect belongs to radiation.