Yesterday in the comments the sevara was thrown out that the criteria of being able to appoint a shliach is required to make a tnai because it indicates control over the consequences of an event. I like the sevara (I think I have heard it in the name of R’ Chaim), but I’m not sure it helps answer R’ Akiva Eiger’s question from neder. It was suggested that since one must personally fulfill the neder one is obviosuly the ba’alim over the ma’aseh. But were that true, I don’t understand the gemara. In every case that one cannot appoint a shliach, why wouldn’t one say that it demonstrates that one is personally the ba’al hama’aseh – what then does the gemara mean to exclude?
The Pnei Yehoshua (Gittin 75) takes almost the opposite approach. Under normal circumstances a verbal declaration cannot undo an action – ain dibur m’vateil ma’aseh. The reason tnai can “undo” an action by making it dependent upon some condition is because the action is not truly complete yet– there is a lack of ba’alus! The fact that something can be turned over to an agent proves this lack of relationship between fulfillment and the personal actions of the ba’alim. But this criteria is necessary only when we are speaking about undoing actions. Words can without question undo other words, and therefore, when it comes to nedarim, nezirus, kiddushin, etc., the criteria of shlichus does not apply – there is question that the words of a tnai can “undo” the words that create these circumstances.
This is not the last word on the topic, but I have not finished my homework on it yet. R’ Chaim in the stencil has a different answer, and the GRI”Z (al haRambam, beg of Hil Nezirus) explains at length the difference between Tos’ approach and that of his father.