The gemara (Bava Metziya 12a) discusses whether a kinyan chatzeir can take effect on an object which does not come to rest, e.g. a ball is thrown through someone’s yard and is caught by someone else after it passes through - is the ball now owned by the owner of the yard or by the one who caught it?
Rashi adds a detail (perhaps based on a girsa which we don’t have) to the example case that should put us on the alert: the thrower was mafkir the ball.
The gemara attempts to resolve this question by citing an analogous case. If someone tries to be makneh a present to his neighbor by throwing the object into his yard, even if the object does not come to rest and cannot be caught as it passes through, a kinyan chatzeir takes place.
Thanks to the detail added by Rashi, these two cases are not as analogous as they may seem. In the case introduced by the question, the thrower surrendered ownership of the ball by being mafkir it upon release. In the case the gemara presents as analogous, there is a da’as acheres makneh – there is intent to give the object specifically to the owner of the chatzeir. How can this be a proof to resolve the original question?
In other words, as Tosfos presents the problem, in the second case, the chatzeir is being koneh an object directed to it by a thrower. In the case used to illustrate the question, the object which is caught or which passes through the chatzeir is an object which is hefker and which therefore lacks any da’as makneh directing it.
My son was nice enough to alert me to a Ketzos (273:1) which opens a window into the whole sugya of hefker through his analysis of this issue, so stay tuned.