Use IRF540 MOSFET to control LED strip
I found a different supplier with a clearer view of suspiciously the exact same board.
  Looks like the input feeds into an optocoupler, allowing the load side supply to pull the gate high. There's a non-switched positive input on the control side that seems to be there only to allow an LED to indicate that the control input is on. Should work without that connected though.

The output side is then entirely isolated and has two power connections at the top terminal block, the positive is commoned to the load block, and the FET switches the low side.