I believe it’s something like dropping 2 SRS levels, so high level items don’t go fully back to one. But it drops with every mistake. So if it’s a persistent leech and you get it wrong more than once in the same review session, it’ll quickly be back at the front of the queue.
Flaming Durdles is an app version that a user here on the site made. 
[Android] Flaming Durtles - Android app with offline support