Knob Cutters are a handy Bonsai Tool for cutting knuckles and other areas of the tree where you want the tree to heal flush. Bonsai-En Has Great Quality Bonsai Tools Cheap.
Knob Cutters or Concave Cutters as they are sometimes referred to are a pretty simple tool so this shouldn't take too long to explain. Sometimes in Bonsai when we make a cut to a trunk we can often times get a knuckle where it heels if you cut flush with the tree. As the Cambium heels over it creates a Bump and sometimes this is unsightly. To prevent this from happening we have Knob Cutters! basically after you have made your cut with your branch cutters you can then go in with the Knob Cutters and create a Concave cut ( Cutting just past Flush ). What this does is allow the tree to heal flush or flat by creating room for the Cambium to heal over