All the above is good advice. (Even the seeming contradiction between Joe (vertigo2u) and Kevin (KaoS) - In his video Joe may be moving his forearm fast but it starts horizontal and ends downwards - so the speed that is being applied to the line builds up).
Some people seem to figure out the multi-lazy fairly quickly. A few others, myself included, take many years. Along the way we discover many different ways to go wrong.
Quentin, could you please do some more sessions and report back with how you get on with the advice so far? If you get it - brilliant! If you have lots of trouble I'll write in some detail about many of the things that can go wrong (e.g. Depth of backflip. Lazy: flipping out on its belly, flipping out on its back, not rotating all the way)
and what to do about them. But I don't want to do that yet, in case it confuses you, and you would have got it fairly quickly without the distraction of my words.
By the way, of the kites you fly, I don't have much experience with the Hypnotist, but had several summers flying mostly QPros - found it best to practice tricks without their weight set. - I think that many people describe them as "neutral" and that is a good description of their feel compared with many other kites. They take a bit more time and air space to flip onto their backs than many trick kites, but otherwise they still do fine.
Joe, your multi-lazies are marvelous now! - Mine still loose altitude...