The horn relay has a coil (a coil of internal wire) that operates (magnetism) the contacts to operate the horn.
There will be positive (12/6 VDC) attached to one side of the relay coil (via a ignition switch other other circuit).
You will measure positive (12/6 VDC) on the other side of the coil because it is a coil of wire.
If you ground that side of the coil it will make the relay operate.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."