How does demotion work?

I was silver 2, I played 3 ranked games this evening. I started at 25 points, lost 15, Lost the next game which cost me 10 points and took me to zero, lost a third game in a row and was demoted to silver 3.

I thought there was like a, I don't know what to call it, Buffer ? that prevents you from being auto demoted on one loss at zero points. I seem to remember it would take a couple of games at zero before you would be taken down a level. Any clarification would be great, or if this is posted elsewhere I'd be happy to read.

Not crying, just curious

Thank you for any responses!