We know that on Rosh Hashana, one is obligated to do the Mitzva of Shofar. But what is the Mitzvah of Shofar? Is it to blow the Shofar? Is it to hear the sounds of the Shofar? Or maybe, is it both?
According to the Rabbeinu Tam, the Mitzva of Shofar is the blowing of the Shofar, since he holds that Asiata Hi Gemura, the action of the Mitzva is the end of the Mitzvah. In other words by blowing Shofar, you have completed your obligation of doing the mitzvah of Shofar. Now this sounds like a good answer. However, if the Mitzva is to blow the Shofar then everybody should have to blow Shofar for themselves?!
The Rambam on the other hand, offers a completely different opinion. He holds that the mitzvah is to hear the sound of the Shofar. He therefore holds that even if the Shofar being used is stolen there is no problem of Mitzvah Habah Baaveira, a Mitzva that comes through a sin, since the Mitzva is to hear the Shofar and not to blow it.
In order to understand the opinion of both the Rambam and the Rabbeinu Tam we need to understand two concepts. One is called Kiyum Mitzvah, fulfilling the Mitzva, and the other is Maaseh Hamitzva, the performance of the Mitzva. These two concepts apply to Shofar and it is only with these two concepts that we can perform the Mitzva of Shofar. It might be that both the Rabbeinu Tam and the Rambam are correct. The performance of the Mitzva is the blowing, and the fulfillment of the Mitzva is the hearing. And it is with another concept known as Shlucho Shel Adam Kimoto, a messenger of a person is like the person doing it, that we can be יוצא the blowing part of the Mitzva through the Baal Tokea while we fulfill the hearing part of the Mitzva by listening with our own ears.