First of all, there's no firmware password or anything fancy. I'm trying to diagnose a late-2011 MacBook Pro 15", and I'm trying to get into the single user mode. I press and hold Cmd+S (the left Cmd, if it changes anything) but I'm not getting into the mode. I've tried pressing (and holding) it before pressing the power button, at the same time, a little after etc. I've tried all the combinations. It just won't enter. It will keep booting or let's say (try booting) into the OS where it gets stuck. When I press Cmd+S, however, I can see a very brief terminal in the middle for a very short/split second, and it disappears as I haven't pressed it all. What am I doing wrong?
UPDATE: I've tried Cmd+S on another 15" MacBook Pro from 2011, and it behaves the same: It just won't let me into recovery mode. They are both Yosemite, one is 10.10.3, and I can't tell the other one's minor version since I can't boot into the system in any way (and it's owner doesn't know the version either).
UPDATE 2: As Buscar웃 asked me about the verbose mode process, I took a video of booting into verbose mode: https://s3.eu-central-1.amazonaws.com/downstage-eu0/IMG_1942.MOV
UPDATE 3: Here's a GIF version of the video. I had to reduce quality and size a lot, but you'd get the general idea: