Verse Dt18-03e describing the gifts given to the Priests by the Jews states And this shall be the priests' due from the people, from them that offer a sacrifice, whether it be ox or sheep, that they shall give unto the priest
• the shoulder, and
• the cheeks, and
• the maw.

Rashi symbolically interprets this gift as affirming the original act of zealotry by Pinchas which earned him eternal Priesthood. Recall that Pinchas speared a couple having relations openly in public and thereby assuaged God's anger at the promiscuity with the Moabite women.

There is a psychological point here: Pinchas killed them because he saw the act as premeditated and malicious, not as an act of passion. The reason he saw the act as premeditated is because, as a priest, he was aware of how things develop. Zimri should have stopped the relationship as it developed. The gifts given to the priest symbolize the development of sin:
• The shoulder [exposure]
• the cheeks [liquid secretions]
• the maw [relations]

Advanced Rashi: Rashi does not literally say what we have said. He corresponds cheek with mouth and prayer and corresponds hand with thrusting the sword. However we feel that Rashi was simply being discrete here. He did not want to explicitly speak about such things. He left it to the reader to infer it. Also Rashi wanted to explain that besides the description of the development of sin the priestly gifts also symbolized the act of zealotry which came about through prayer and using a sword.