As my guitar player calls it a chick and a hssp.
Closed hihat sound depends on how much you close it with your feet but if you fully close it you will get a distinct sharp stroke. It is comparable to a ride cymbal sound structurally. Then slowly releasing your feet from the clutch and the sustain slowly increases from very clean and sharply decaying sound to more sizzling sound.
Hihat footplay (ehm, no pun intended) is more of a orchestral cymbal playing. You can see those dudes are holding the hihat completely in the wrong way. Look ;p (click the image for the source)
So in this analogy you can just crash both and keep your hands tight. That would be the typical jazz offbeat sound. Also you can crash and pull your hands apart as fast as possible. The most often usage for this is when a drummer is playing a roll and increasing the tension by counting the beats with increasing volume of stomps, one of my favorites
Steve Gadd is a true master of many things but especially command of hihat with foot.