Example 11.10.7.4. Let $f: X \rightarrow Y$ be a morphism of simplicial sets. It follows from Proposition 4.4.2.1 that the following conditions are equivalent:
The morphism $f$ is a covariant equivalence relative to $\Delta ^{0}$.
The morphism $f$ is a contravariant equivalence relative to $\Delta ^{0}$.
The morphism $f$ is a weak homotopy equivalence.
See Proposition 11.10.7.16 for a more general statement.