How physics constraints work?

Hi,

I have some trouble to understand how physics constraint works, especially the Linear Motor part.
In this exemple I’m trying to find a way to link the sphere to the cube, and have a force pulling the sphere towards the cube, like a grappling hook.
But as you can see in the video the Linear Motor doesn’t seem to work :

Thank in advance
You can find my set up here :



Official constraint documentation: