I can't claim to have discovered the technique, I pinched it off that other thread I mentioned earlier...
Bad example of what it can do as it was underexposed by about four stops or more and pushed up three in post-processing: (first go at macro so it's not exactly a good example of what one could do... this watch is about 18-19mm wide)
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I was fiddling in a dark room, this is my nurse's watch.. metering is all over the shop with this technique, I should really meter through the lens then put the camera in manual while it's off. I'd be interested in getting a mount to attach one lens to the other and getting me a nifty fifty again to whack on the back and not worry about the rear element of... or maybe a reversed lens hood..
Anyway, that photo only turned out okay but I'm sure with some more lighting and proper exposure I could get some quite good results of this, the other shots that I did that aren't uploaded are really quite sharp though the DOF is ridiculously thin.

Thanks for the replies too, guys, it sounds like it's not going to be damaging anything...