What are they exactly? I know they are vacuums...I saw two types in the store. One was a siphon while the other claimed not to be a siphon. I already have a siphon, so I was thinking about getting the non-siphon one, the one that "operates on water pressure". Just wondering if any of y'all have that one, how it works, all of that. The Python site and the store clerk didn't help.