On my crimping tool, if you look closely in the valley, you will see that it is really two side-by-side crimp areas. One side has a small peak inside the valley. the is used to turn the prongs into the wire. the other side does not have this peak and it is used to wrap the prongs around the insulated part of the wire. So I crimp just the exposed wire portion first (using the side of the tool that has the peak). then I crimp the portion over the insulation.