Thanks for posting. I have never seen that. Where does the lamb come from?

That is "the Star" fracture on the lamb; you can perform that on both the fore shank and the hind shank. You hold the shank and product very firmly with one hand with the shank bone hanging over the table edge and grip the shank bone just past the elbow or knee joint and 'snap' downward and break off the bone; it will fracture into that 'star' pattern. It is the preferred way to dress up a leg of lamb presentation, an old chef's trick. (my dad was a meatcutter back in the 30's and 40's and when he went overseas on the troop ship a couple cooks got sick and he had to prepare 400 legs of lamb for dinner one night, boning them out except for the shank bone leaving the star joint on the shank to grip them with and as presentation to carve; a chef from the Waldorf Astoria showed him that who was conscripted on ship also.)
You have to be careful, however; one time I was snapping them off at an IGA I worked for and got tendonitis real bad in my wrist when one didn't snap, it sprang back on me and snapped my wrist instead!