The gemara says that one is able to make a moda'ah on a gett and a gift by notifying witnesses in advance that he does not want the gett or gift to go into effect. The Rashbam writes clearly that the moda'ah doesn't really accomplish anything more than notifying the witnesses that he is being forced into this transaction. The moda'ah itself doesn't prevent the gett or kinyan from taking effect, rather it is the o'nes, the fact that he is not writing the gett of his own free will that is really mevatel the gett. The purpose of the moda'ah is just to let witnesses in on the fact the he is being coerced into giving the gett. Therefore if one would give a moda'ah on a gett that he is merely trying to tease his wife, and then writes her a gett, the rashbam holds that the gett would be binding since there wasn't any o'nes. The reason why the moda'ah itself cannot nullify the gett, the rashbam explains - אנן סהדי דגמר בלבו בשעת מעעשה לעשותו גט גמור. Meaning, just as we say that if one is explicitly mevatel earlier moda'as, the later statement is binding. Here too, we consider the action of giving the gett without duress, to be a bitul of the earlier moda'ah.
The Rosh disagrees with the rashbam and holds that one may give a moda'ah even without an o'nes. Furthermore, the Rosh says that if one gave a moda'ah and at the time expressed a particular reason for his moda'ah, which we later found out to be false, the moda'ah would still be effective in nullifying the gett or matana. According to the Rosh, a moda'ah is powerful by itself to be mevatel the gett, even in the absence of o'nes.
The point of argument between the Rashbam and Rosh is not about the power of a moda'ah, in the absence of an o'nes. All would seem to agree that even in the absence of o'nes the moda'ah technically has the ability to be mevatel the gett. The machlokes seems to be whether we consider the motions of going through a gett to be a bitul of his original moda'ah. The Rashbam holds that the motions of giving a gett is a bitul to the moda'ah, whereas the Rosh holds that to be mevatel the moda'ah one must do so explicitly. Based on this, in a case where one did not follow up the moda'ah with an action of giving the gett, rather the gett was written and delivered to her by an agent that he initially appointed (and the ba'al was never mevatel the shelichus) - If the ba'al initially gave a moda'ah on the gett that he or his sheliach would write and give, even in the absence of an o'nes, the gett would be void even according to the rashbam. The rationale would be that in this case the ba'al never did the action of writing and giving the gett himself to serve as a bitul of the moda'ah, therefore the original moda'ah retains the power to be mevatel the gett.