I got 5 end mills from eBay - 6mm Straight Shank 3mm End Mill Bit for Aluminum Iron | eBay

However they don't seem to have any teeth that go all the way to the centre of the cutter. I expected that 1 of the 3 teeth would do. Does this mean it can't plunge? Why does it have this rather deep slit on the cutting end?

They call it a slot drill so I took that to mean it will plunge.