neither
Basically it has a little one way floating valve in the bottom, just push it into the liquid and it fills. The prongs on the bottom are to make it harder to press the stem of the valve once you have your sample out of the fermentor (i.e. make it harder for the sample to end up all over the counter). You can't really just put a finger over the hole because the opening is big enough for you to put your hydrometer inside negating the need for a sample tube. In theory you can then press the valve stem against the lip of the carboy/bucket to return the sample. But that violates the rule that your sample should be drunk after you get the hydro reading. It's a federal law, so you have to do it...
Side note for anyone that has broken a fermtech auto-siphon. Can you use the outer part by itself as a wine thief? Is it big enough diameter to float the hydrometer in?
Edit: just to clarify, it might look like there are three prongs there. The outer two are just to guard the little stem you are intended to push to release the sample.