Thus, ST polarization can be spin-locked by an on-resonance effective rf field at a direction defined by the phase of the complex ST spinning sideband sn near the rf frequency φn. The nutation frequency about the effective rf, relevant to the CP, is scaled down by |sn|. The spinlock enables CP from protons to the pair of two-level ST systems of S = 3/2 quadrupolar nuclei when the HH condition is matched with the scaled nutation frequency. The direction of the spinlock and the generated CP signal phase varies widely. A powder average leads to signal cancellation and low efficiency. The situation is different for 1 H → X → 1 H double-CP because the effective rf phase is the same between the two encoding pulses, and the phase spread does not lead to signal cancellation. This is the key mechanism behind efficient double-CP for HETCOR of quadrupolar nuclei.