  Proton elliptic flow is studied as a function of impact-parameter $b$, for
two transverse momentum cuts in 2 - 6 AGeV Au + Au collisions. The elliptic
flow shows an essentially linear dependence on b (for $1.5 < b < 8$ fm) with a
negative slope at 2 AGeV, a positive slope at 6 AGeV and a near zero slope at 4
AGeV. These dependencies serve as an important constraint for discriminating
between various equations of state (EOS) for high density nuclear matter, and
they provide important insights on the interplay between collision geometry and
the expansion dynamics. Extensive comparisons of the measured and calculated
differential flows provide further evidence for a softening of the EOS between
2 and 6 GeV/nucleon.
