The dull knife is because you are cutting length wise through the chicken breast into thinner pieces and if you had a sharp knife it could cut your hand.
You WILL have the blade at some point contacting your hand cutting a chicken breast this way, so use maybe a not so sharp knife works best.
However, my friend, you are still wrong..a dull knife is inherently dangerous..that's what causes accidents. The best way to butterfly a breast is to put it in the freezer for 30 minutes BEFORE cutting it..far easier to cut..or if it's frozen, do not defrost completely before slicing..