You need to add value + to the expression so that the original position of each layer is maintained. Just go to the front of the expression and type in value + before all the other language. That is all there is to it.
Also, the original tutorial didn't show you the most efficient way to apply expressions to a bunch of layers. You use the Edit menu and copy expression only, then you select all of the layers and paste. You'll end up with more predictable results. I also think it's a better idea to attach your expression controllers to a null because null layers are never examined when rendering and adjustment layers are. Even if there are no effects in the adjustment layer it takes cycles to look at the layer and in complex comps this can start slowing things down.
Where exactly shoud I add value +???
I am total beginner in After effects, I start to learn after effects 5 days ago
Can you send maybe a print screen, where exactly shoud I type it?
Select the x property of one of the segments, hold down the alt/option key and click on the stopwatch to add an expression. Type "value + " then use the pickwhip to connect to your x position slider. That's it.
Now highlight the x position and go to the edit menu and choose copy expression only. now select all of the other layers on that side of your robot and paste.
That should do it. The expression should read something like:
value + thisComp.layer("controller").effect("Slider Comtrol")("Slider");
You are my GOD