There’s no trick that I know of.
My guess is that he’s got a parts gun and the safety doesn’t quite match up with the cocking piece (the plug-like part attached to the rear of the firing pin). Specifically, when the cocking piece is in the full rearward (cocked) position, it still isn’t quite far enough back to allow the safety to pivot to the ON position.
If this sounds likely, try the following.
Remove the bolt from the receiver. Note that the firing pin / cocking piece assembly is about ¾ cocked when you do this. (Pushing the bolt forward in the receiver normally finishes cocking the assembly.)
Put the bolt horizontally but upside down in a padded vise. It needs to be fairly snug in the vise, but of course don't bend anything.
With a good sized screwdriver against the cocking piece’s bottom, front lug (where the sear normally engages), push the cocking piece further rearwards while applying gentle pressure on the safety trying get it to engage.
Don’t slip with the screwdriver!!
With luck, once you get the cocking piece about 3/16ths of an inch further back, the safety will engage.
Good luck!