If I am not mistaken, the opening, like a keyhole, would allow the top of the can to slide into the larger portion - then slip the can forward into the narrow area. The pressure itself would hold the can. The trigger piece, would have to tip forward, toward the can to allow the top portion to come to rest on the nozzle. As you pull the trigger, the top portion of the metal "trigger" would depress the valve at the top.
That is my "guess." It sure looks like the ones that I have used for that purpose.