On heating the precursor is rapidly iodinated by the hydrogen iodide to form iodoephedrine.
From the en.wikipedia.org
Because hydrogen iodide exists in a chemical equilibrium with iodine and hydrogen, the phosphorus reaction shifts the balance toward hydrogen production when iodine is consumed.
From the en.wikipedia.org
The team looked at the alignment of the spins of carbon and hydrogen nuclei in molecules of methyl iodide.