To open.
Open up the door!
To reveal oneself; to share personal information about oneself; to become communicative.
After three hours of questioning, he finally opened up.
To commence firing weapons.
As the convoy entered the pass, we opened up on them with everything we had.
(of the sky) To rain.
The sky has really opened up. I've never seen this much rain!
To begin running, driving, travelling, etc., at maximum speed.