more sense to have sandbagging trace back to sandbags rather than socks

Sock it to me straight, mate, I won't have any truck with shady sandbaggers.

There's something sneaky and surprising about sandbags - they hit a lot harder than you expect (can even knock you senseless), and don't leave a mark.

But I suspect there is something in Alex's theatre theory - if you have sandbags suspended at a great height and they fall down when you're underneath and not prepared , you'll really know about it (assuming you're not dead). In a large theatre it would take experience to know where the sandbags are and where they're going, and thus to work safely backstage.

Duck!