A knot that can be tied while the line is taut.

[knot4u]

I like the concept of the spring.  Look, if this wire has no give like basically all wires have no give, then this wire is going to be rather tight or just loose.  There is no in between.  There's no use in trying to make things as tight as possible because a couple of millimeters of slip would make your wire loose again.  A spring would install some give to fool the wire into believing it's a rope so that the wire can be an overachiever.

Anyway, bloooooop, do you find the wire has no give to it?  Or am I off?

[blooop]

I initially used fishing line which had some give, but went to wire because of its lack of give.  In the base, the pulleys have a non-circular profile, which only works if the line has no give at all.  If there is any give in the wire system, the wires on the opposite side go very slack and fall out of the pulley groove.