I was reading the paper below, and because of my lack of knowledge on the linguistic terms, I have been stuck half way through. If you would be kind enough to enlighten me, I would be very much obliged. My take is 'existential force' points to, as it says, the phenomenon where the referent is in existence whereas 'universal force' points to the force that a free choice item has.
Incidentally, if you would also paraphrase the 'when it gets bound by an unselective operator' into lay person's terms I would also appreciate it.
[E]xamples like (10) indicate that weak definites have the force of the complement indefinite not only when the indefinite has existential force, but also when it gets bound by an unselective operator and acquires universal force:
(10) If the student of a linguist owns a donkey, he beats it.
A different solution comes to mind, namely, that the reason why weak definites have the interpretation we are interested in is because their interpretation gets 'anchored' to that of the NP that serves as complement of of, so that if that NP gets bound by an operator, these definites get bound by that operator as well.[[Weak Definites] by Massimo Poesio]