This is a great little tool for cutting leather laces. It's called an Australian Strander.
To use, place the strander along the edge of your leather and pull back towards you with your right hand, while maintaining a firm pull with your left.
The strander works equally well on curved or straight edges and each blade can be reversed once blunt, so both ends are used before the blade is replaced. This tool uses the same super sharp replaceable blades as our safety beveller and super skiver.
As soon as you notice resistance when cutting, it's time to change your blade.