yes you can do it that way, it takes a while though. the coral has to build a skeletal structure onto the other rock.
what i like to do is to get out the dremel with a diamond edged saw type blade, and cut the coral between the polyps to the desired
frag size. you will want to add a couple of polyps to the
frag size, because usually new
frags lose a polyps or two due to the heat from the blade rubbing against them and creating a little tissue damage.
i like to run the dremel saw blade to a depth of about 1/2 of the skeleton of the coral, then around the same section on the bottom creating a grove on the bottom (that makes for a cleaner break), and then break the skeleton with your hands. be sure to grab it as close to the bottom of the skeleton as possible so you do not damage the remaining polyps.
before fragging i also try to get the coral to close its polyps as much as possible so you can see where you want to cut it.
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