aside from the above meanings, let it rip also has the meaning of 'to fart' lol

let it rip / let her rip 1. to make a vehicle or machine move very fast He would get up early Sunday mornings, fire up the lawnmower, and let her rip. Usage notes: the pronoun her is often used to refer to a machine 2. to do something with energy and enthusiasm Hendrix would walk into a recording studio, turn on his amp, and let it rip while the tape recorder rolled. http://idioms.thefreedictionary.com/let+it+rip

It means simply "Just do it", usually when there is some form of uncertainty. For example, if you just repaired some ball bearings in an engine and you are not sure if the engine will start, then you might tell your friend "Well, Let her rip!" Beware it is slang and not often used anymore, at least in the US.

Rupert: I'm very sorry old chap, but I think I'm going to fart. Oliver: Oh, let it rip, old boy! Let it rip!