When one correctly performs a rattlesnake, they will have succeeded in poking the assailant's eyes (please refer to The Three Stooges for more information) and also biting them in the scrotum (if they happen to be male).
There is room for variation and interpretation, but preferably, one will not actually have to go through with the threat; the assailant will be confused with your random combination of threats, or scared for their lives.
Tom: "What's a rattlesnake...?"
Fred: "It's where I poke you in the eyes and bite your scrotum!"
Tom: "Woah! Weird! Take your hat back and never talk to me again!"