It comes from something we see often on corals that have been rolled in holding tanks on rough days or have been knocked during collection. basically all you need is a little bit of loose skeletal material along the out side edge of the coral. gravity will do the rest with a little time. The weight of the corallite slowly stretches down ward and finally gives way. you can glue the resulting small frag to a disc and it will encrust the disc really well, unlike other methods.
How you go about it depends on how confident you feel you can get in with some bone cutters and give a little snip at the edge of the corallite or give it a little bash with something more solid. You do want the breaks to be at the lip so the loose bits hang down. Strong water flow can carry away the bits but usually your elegance should be in a less dynamic area of the aquarium.
Regards
Peter Fullarton
Oceanarium