The gemara in several locations allows a person to execute a verbal mitzva (such as the recitation of berakhot) through the principle of shomei'a ke-oneh. Namely, instead of actually reciting the berakha, a person may listen to its recitation by another (ideally answering amen) and thereby fulfill his mitzva. This shiur will explore the root and basic nature of this principle.
A fundamental machloket between Rashi and Tosafot sheds light upon the nature of shomei'a ke-oneh. In two locations (Sukka 38b and Berakhot 21b), they debate a situation in which someone is 'stuck' in the middle of shemoneh esrei while the tzibbur is reciting kedusha. Rashi claims that the person should silently pause his shemoneh esrei and listen to kedusha. Based on the principle of shomei'a ke-oneh, he can perform his recitation of kedusha without actually reciting it. Tosafot disagrees, claiming that by participating in kedusha through shomei'a ke-oneh, one is considered as having interrupted his shemoneh esrei by reciting kedusha. Apparently, Rashi and Tosafot adopted two very different perspectives on shomei'a ke-oneh. Tosafot took the term literally – one who listens to another person's recitation is considered as having recited the text personally. In the context of shemoneh esrei, listening to kedusha would be equivalent to actually reciting kedusha and would constitute a hefsek. By contrast, Rashi believed that listening is just a means of having another person execute the mitzva on one's behalf (through some shelichut equivalent). It does not constitute some alternate form of personally reciting the kedusha and is therefore permitted, since no interruption of shmoneh esrei will occur.
This machloket between Rashi and Tosafot surrounds their interpretation of the letter 'kaf' - known as the "kaf ha-dimayon" (the kaf of comparison). When the gemara claims "shomei'a KE-oneh," "listening is LIKE reciting," is this comparison to be taken literally, that listening is deemed by halakha as an additional form of recitation? Or is this statement to be read 'figuratively,' that one who listens has fulfilled the mitzva in a complete manner 'as if' he had actually recited the given text?
Interestingly enough, the gemara cites a source for shomei'a ke-oneh which more convincingly reflects Tosafot's view. Melakhim II perek 22 describes the episode of the Sefer Torah found in the Mikdash during the reign of Yoshiyahu. His scribe Shafan read the Torah to Yoshiyahu, prompting a nationwide teshuva movement. Pasuk 16 describes this Torah as the sefer which Yoshiyahu read - even though he only listened to the reading conducted by Shafan. This does indeed suggest that listening to a reading is equivalent to personal recitation.
A second potential consequence of this question surrounds the ability to fulfill the obligation of keri'at ha-Torah through the mechanism of shomei'a ke-oneh. This issue was posed through the question of whether a blind person can receive an aliya. The Ya'avetz (Rav Yaakov Emden - 1:75) prohibited calling the blind for an aliya, while the Taz (Orach Chayim 141:3) allowed this practice. A person given an aliya must read from the text of the Torah – something a blind person is incapable of performing on his own. Can someone read on his behalf while he listens, such that we can consider this a valid reading based on the principle of shomei'a ke-oneh? If shomei'a ke-oneh allows us to attribute Reuven's recitation to Shimon, we might conceivably agree with the Taz, that the blind person can adopt the reading of another - a reading which was performed from a text. By contrast, if shomei'a ke-oneh defines an act of listening as a personal recitation, we cannot validate the blind man's listening in this instance, since he is not 'viewing' a text. Non-textual statements can be validly articulated through the act of listening, but statements which must be read from a text cannot be executed through shomei'a ke-oneh.
A structurally similar question was debated by the Beit Halevi and the Netziv. Can a kohen perform the mitzva of birkat kohanim through shomei'a ke-oneh? The Beit Halevi claimed that as birkat kohanim must be recited in an audible voice (based on the gemara in Sota 38a), shomei'a ke-oneh will not be effective. Assumedly, he viewed shomei'a ke-oneh as an alternate form of recitation; listening is comparable to actually speaking. In this instance, though, the form of speech simulated by listening is insufficient, as no audible voice was heard. By contrast, the Netziv viewed shomei'a ke-oneh as a manner of embracing someone else's recital as one's own. By listening to other kohanim recite the berakha, the 'silent' kohen assumes their loud and audible recitation as his own, thus executing a valid performance of birkat kohanim. The Chazon Ish, agreeing with the Netziv, cites an interesting proof to the fact that shomei'a ke-oneh does not constitute an alternate form of personal recital. The mishna in Sukka (38a) demands that shomei'a ke-oneh be performed through the agency of someone who is obligated in that particular mitzva. Hence, a katan (minor) cannot recite hallel on behalf of a gadol (adult), since he himself bears no obligation (by virtue of his age) to recite hallel. If shomei'a ke-oneh renders listening to a recitation a form of actual, personal recitation, why must the source recitation be conducted by a person who is obligated in that particular mitzva? By listening to a katan recite hallel, why can't an adult be considered as having actually recited hallel?
Based on this gemara, shomei'a ke-oneh would be seen as a manner of aligning yourself with someone else's performance of a mitzva.
That someone, performs the mitzva on your behalf and must be obligated in the mitzva.