I sprinkle it around and then work it in a little, more to deter dogs than for the garden. The other day I was sprinkling bone meal around and turned around to see my dog was following me a licking it off the surface of the ground!
My understanding is that bone meal releases slowly, and I believe the nutrients need to be in solution form for plants to access them, so rain will gradually dissolve the fertilizer and bring it to the root zone.