Pitching on a yeast cake

Okay, so I'm reading posts about people pitching onto an existing yeast cake.

So how does one do this? Just siphon your wort into the carboy? What about cleaning and sanitizing.... I'm so confused

You are not confused...you just think you are because it sounds so unconventional.
What you describe is exactky what I did when I knew I needed a very large "starter" for my Imperial Stout.
I had only minutes before, siphoned my brown ale out of primary and then, in went the stout wort.
Let me tell you...there was No delay in bubble activity. I saw and increase from about 5 bbles/ min to about 10 in the first hour.
If you think about it, "sanitizing" is not an issue since presumably it was sanitaized in there to begin with. Technically, it is not sanitized since it is teeming with eleventy billion hungry yeasties that are just waiting for there next meal.
So, yes. Go for it.
Just consider if it is the same strain of yeast that you want to use and after a very high gravity primary (1.08+), the reamining yeast may be a bit too pooped to fire up again. Also youve got some hop pellet gunk in there too but since most are doing the trub pitch because they want an etreme yeast count, it often means that they are making something really big and that lefft over hops in there wont impact much. Thats what I went with anyway.
Hope that helps...

Don't worry about sanitizing, the beer you just siphoned off was in a sanitized carboy.
I would do this as soon as possible after you siphion off the first beer though.
I recently tried this for the first time and the results were great. I brewed a Cream Ale and then put a Brown Ale on top of the yeast cake. oxygenate well.
The air lock was working before I got the brewing equipment cleaned up and it fermented strong for about 4 or 5 days. The resulting Brown Ale is one of my best beers to date. And it was like getting the yeast for free.
I have been told you should do the same type of beer or one stronger ( darker, hoppier) not the other way around. as the second beer may take on some of the first beers flavor. I don't know this from personal experience but from those in my club. Hope this helps. Oh and I have also been told that about three times is the recommended max.