Dave,

Door etc frame fixings work in this way - you drill through the frame and
then the wall at the same time, and hammer a special fixing through them
both. It give a more accurate location than doing it separately.

I've come across this technique with exactly that sort of special fixing;
what I hadn't realised was that it's also done with standard wallplugs.
Thanks to everyone who put me right.

Bert
http://www.bertcoules.co.uk