The red p or phosphorous content eats the iodine and hydriodic acid is the product, along with (I think?) some stuff that gets recycled (hypo?), phosgene gas, and some water?
It is not necessary to make meth if you already have the HI (hydriodic acid), but it recycles and keeps reacting with any iodine until it is exhausted. This is why you can save your red p and use it for a few cooks before it is inactive (or so I've been told). I've never heard of anyone being able to save the hypo for another use, so I'm not sure about that. The red p usually gets filtered out of solution after the reaction with EP/PSE and any leftover iodine gets neutralized with sodium thiosulfate before turning it into HCl.
I don't know the exact mechanism and don't feel like looking for it any longer.... I haven't seen too much info about this either because it is an 'outdated' method and nobody can be bothered anymore, so I'm not exactly sure if the red p will regenerate the hydriodic acid after it has been used to reduce EP/PSE or in-situ or if it can only be used to react with iodine to make hydriodic acid. Perhaps someone who really knows and can explain it will indulge us...