spamlet has brought this to us :
What is it that makes 'SDS' able to do a job that a 'hammer' drill won't?

A very much harder impact on the tip of the bit. In the old days,
before SDS, when hammer drills appeared - If the hammer drill would not
make any headway in the hole, you would fall back onto a Rawldrill. A
hard triangular drill bit, in a holder which you hit repeatedly with a
hammer, turning slightly between each strike. They would drill through
anything, but it was hard work and took time. The SDS takes a similar
approach, a very hard blow on the tip of the bit combined with
comparatively slow rotation.

A hammer drill combines fast rotation with a much lighter blow on the
drill bit. Fine for material which is not so hard.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk