Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of Active Charge Exchange Spectroscopy and the availability of multi-frequency ICRH with 3He minority. The determination of ion threshold and stiffness offers unique opportunities for validation of the well-established theory of Ion Temperature Gradient driven modes. Ion stiffness is observed to decrease strongly in presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known wE¥B threshold up-shift and plays a major role in enhancing core confinement in Hybrid regimes and Ion Internal Transport Barriers. The effects of Te/Ti and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simu-lations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.