  Within the ultrarelativistic quantum molecular dynamics (UrQMD) model, by
reverse tracing nucleons that are finally emitted at mid-rapidity (|$y_0$| <
0.1) in the entire reaction process, the time evolution of elliptic flow
($v_2$) of these traced nucleons produced in Au+Au collisions at beam energy of
0.4 GeV$/$nucleon with different impact parameters ($b$) is studied. The
initial value of $v_2$ is positive and increases with $b$, then it decreases as
time passes and tends to saturate at a negative value. It is found that
nucleon-nucleon collisions always depress the value of $v_2$ (enhance the
out-of-plane emission), while the nuclear mean field potential may slightly
raise the value of $v_2$ during the expansion stage in peripheral reactions.
The related density mostly probed by $v_2$ of nucleons at mid-rapidity is found
to be $\sim$ 60% of the maximum density reached during the collisions.
