why do tack hammers have a split on one peen?

One end of the hammer is a magnet, without the split it would go dead with a few blows. If you hit a magnet on one end it will go dead or become very weak. I don't know how the split prevents this, but it does. In concept the magnet is like a horse shoe. The bottom or rounded end is not magnetic.

Tack Hammers

The side with the split in it is actually a magnet. It holds the tacks for easy nailing. Otherwise, most of us would have to use pliers to hold the tack, to avoid smashing our fingers.

LOL... Why yes, I have used a ball peen hammer before and in fact, I have one in my tool box... LOL... Would you like to teach me how to use it properly... LOL...