I found this video on youtube, and It seems that what this guy is doing when linking turns with the back foot is ruddering.
Yes, he is. He is using a substantial pivot component in his turns.
Is this guy using the wrong technique?
IMO, it's not very elegant. Pivot turns certainly have their uses, but on decent terrain, nice smooth turns are IMO much more fun and look quite a bit better...
To turn toeside, I ease up on the pressure of my front heel, but at the same time keep pressure on my back heel. When the board points down hill, I let up pressure on my back heel, put pressure on my front toe, and then back toe. And same to go back to heelside.
Well, when your board is pointing down the fall line, you should have partially completed the tail edge change (gone a bit beyond a flat tail). As you progress, go faster and get more dynamic, you can do the edge change earlier in the turn and significantly improve turn initiation.
If you focus on keeping more weight on the front foot in the beginning of the turn, you can initiate turns much faster, pick up less speed during turn initiation and you have to do less emergency speed control in the bottom of the turn. Even if you feel like you are leaning over the front leg, you are likely only centered on the board.
When I am on the mountain, when you are going fast, it just seems that these are a lot of steps to do in so little time.
That works automatically with practice.
I tend to twist my hips to initiate the turns. Is this the wrong thing to do?
Hip rotation into the turn does induce the right kind of board twist (enhances foot/torsional steering) and does speed up turn initiation. IMHO, it's not wrong to use it for dynamic, skidded turns.