Hi Elrick,

> Why is the splines in MoI stretching in a symetrical way when I try to move
> it? Which shortcut or button controls this?

It sounds like it's the "History" function at work - after you do a mirror of something if you then edit the original the mirror transform will be reapplied and the mirrored version will update.

If you don't want it to happen you can disable history on that object by selecting the mirrored piece (or just select both pieces), and then run the Edit > History command and push the "Disable update" button that shows up in the command options area.

- Michael