How do I kick an opponent in the head on halo 3?

[Wayne G]

when I was playing xbox live I saw my opponent jump and kick my teammate in the head!!! I thought that couldn't be right but then he did it to me and it's like a 1 hit kill! How do you do that?

 

[KO310]

when you jump in someones head and hit them youll kill them with a assin.(they hit you in the back)