You really don't cut through it, you expose it and run your fingers along the bone and cartilage then lift it out. Come to think of it this may only work with younger chickens, not old hens etc.
I don't know where I saw it but I take out the backbone with poultry shears (sometimes before eviscerating, sometimes after) Then spread them open to reveal the breast bone. If you take a knife and just slightly cut the cartilage right where the wishbone meets the breastbone. Then push down on...