Every spell requires a catalyst of some form or another. In this case, Elm is the catalyst, his precum used as currency in a long standing deal. He is equipped with ritual bindings that magically block all movement, while the charm on his dick denies any orgasms, and greatly increases the perception of pleasure. The charm on his balls increasing his bodies natural production rate.
Once posed and ready, a gateway is summoned beneath him, letting a powerful and lustful creature tease and torment Elm, feasting on the frustration and desperation infused within his precum as it pleases. Once satisfied with its meal, the creature, in return, shares a portion of its power to the high mage, to aid in greater spell casting. This typically leaves the catalyst pent up, exhausted, perhaps with a sore rear, but largely unharmed.

While its common for the average catalyst for this ritual to become so pent up, that their balls physically swell over the course of the lengthy ritual, Elm's condition has a unique reaction. On top of his unnaturally fast production rate, Elm's body is hyper sensitive to any and all spells, potions, or drugs that affect production or fertility, causing them to become several times more effective on him. The casters all look on in astonishment as Elm's balls visibly swell larger than any previous catalyst, in just a matter of minutes.
Unfortunately for Elm, this ritual has only just begun, and commonly takes over a day to complete. The creature, feeling just how much Elm has to offer, may end up taking quite a liking to him. After all, the deal not based on how much it takes, but how much it desires, and this creature is feeling extra greedy. Furthermore, the mages guild now likely sees Elm as a top tier bargaining chip in deals such as this. Meaning it would be of no surprise if he was used in many, many more rituals.

(Don't ask why I spent so long on the lore of a single image. Because I don't know lol)