Extrude Subcurve Min Value Problem

[clapperboy]

Im animating this tube thats is created with a surface extrude.
Im starting with a circle and extrude all along the path then I want the extrude to
when I animate the subcurve max from 0 to 1.
Its working great., the extrude follow the path.

What I was wondering is why when you animate the min value, it goes all crazy instead of following the path.
I would assume min would close down the extrude following the path.

Is there a way i can achieve this effect?

Thanks