This will be the same answer for the other question in the technique section. It's the bell on the hi hats being hit while the hats are closed. If they are open the bell goes "ping", if they are closed the bell goes "clank". The amount of open or closed will determine how much ping or clank you can get out of the bell.
The type/size/brand of hi hats does not matter, only that the top one has a bell. Cymbals aren't usually dampened.
You should try taking up drums. Playing them is way better than writing about them.