Take a coin with a head on it. Put the grove over the raised face with groove side up. Press both ends of the pill down with thumbs. It breaks the pill along the groove.
What are you using to cut it? The trick I found with the pull cutters is to set it then slap it with your hand to make the cut in one go. Also, once the blade is a bit dull, it’s over and you’ll need a new one.
FYI French and never heard of a pill cutter. Used my fingers for the last 30 years and I would estimate the failure rate at well below 1%. It is just that the dent on this one is u shaped, while nearly all other are v shaped, making the snap easy easy. Also pill cutter seriously? Heard of knifes?
Pill cutters are situationally easier for the elderly to use. They're portable, don't require a cutting surface, and generally have guards along the blade making it safer than a knife.
What works for me is snapping it while my fingernail is in the groove. Gives a clean break down the center like every time.
The dent signifies that there's an even distribution of active ingredients across the entire pill. It's a break line but it's purpose isn't for hand-snapping