Contributed by: R. Ezra Mizrahi
Rashi explains that the word "sav" ("command") is used to hasten response and call one to action. Rabbi Shimon adds that when there is a case of monetary loss there is a greater need to specify this exact call to action.
What monetary loss is there?
There are various opinions. The Gur Aryeh explains that a Kohen would need to stop his regular method of earning a living, come to Bet Hamiqdash, and perform sacrifices. The Kohen would lose money especially this case, because with the "Olah" sacrifice, the Kohen does not receive any part of it (excluding the hide).
The Ramban explains that the monetary loss mentioned here is referring to the introductory minha offering that every Kohen must pay for on the onset of his service in Bet Hamiqdash.
Regardless, we can explain this idea of "hisaron kis" or "monetary loss" by exploring the word "kis" (literally meaning pocket) figuratively.
All body parts have a "kis" or covering of sorts. The eyes have eyelids, the mouth has the lips, ears can easily be folded to block sound, and so forth. However, there is one part of the body that is not and cannot be blocked- our thoughts. Our mind is free in the truest sense. It is allowed to think whatever it likes, whenever it likes.
This is where we can understand Rashi, specifically the words of Rabbi Shimon in a novel way. The "Olah" sacrifice was usually brought to atone for one's impure and intentions. One must use language of haste when dealing with such elusive thoughts for they are not kept in a covering- i.e. they are hard to define and hard to atone for, This is why the Torah uses the language of Ssav.