This is a military term for aligning yourself in formation with the person in
front of you, so that when viewed from the
front of a formation you all are covered and aligned correctly.
In the vernacular, to cover down means to get someone's back when they fuck up.
Example: "Where is
Jones, why isn't he here?"
"He's at dental sergeant."
That's covering down.
When your
wife drinks too much, you cover down on that shit, hold her
hair out of the
toilet, and tell nobody about it.