Full Idea
The supervaluation approach to vagueness is to construe vague predicates not as ones with fuzzy borderlines and no cut-off, but as having a cut-off somewhere, but in no particular place.
Gist of Idea
Supervaluations say there is a cut-off somewhere, but at no particular place
Source
Stephen Read (Thinking About Logic [1995], Ch.7)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.200
A Reaction
Presumably you narrow down the gap by supervaluation, then split the difference to get a definite value.