I'm not able to look at the video at this time, but I can tell you the basic steps for using the erg, and provide you with a couple warnings to avoid injury.
Warnings first:
Always, always keep your back straight, from the bottom to the top -- no bananas! Only rulers can row right
Always keep your wrists straight, too -- if you pull with a bent wrist, you can injure your tendons and ligaments.
Ok, so the stroke, and I recommend you try it slowly a few times to get the feel for it, before you try to go all in
:
Sit up at the front of the erg, with your legs perpendicular to the floor, back straight but leaning forward (I'm no good with angles, but maybe 45 degrees?), arms straight and holding the bar.
Push your butt back, keeping your back straight and at the angle you were at when sitting at the front. Keep your arms straight, too. So your butt and the seat are moving back, and your body (and the bar) is moving back with it. Don't move JUST your butt and the seat -- make sure that everything's shifting back simultaneously, but only your legs are making the "boat" move forward at this point.
As you reach the end of your leg push, start to lean back with your back.
As you reach the end of your back lean (remembering to keep it straight!), start pulling the bar toward you with your arms (remembering to keep your wrists unbent -- you can let them turn sideways on the bar to get a better reach, but don't let them bend up or down).
When your hands are at your chest, that's the end of the stroke.
Then start moving your arms forward, staying legs straight and back leaned back.
When your arms are almost straight, start leaning forward.
When you're almost all leaned forward, let your knees come up and slide your butt up toward the front of the erg, remembering to keep your back straight and your arms straight and up.
Repeat!
Please note, I am not a professional or a coach, so if you're feeling pain, stop, etc. These are just rough guidelines for how to erg successfully.
Good luck!