  The AAV effect is the well-known phenomenon where a weak measurement followed
by post-selection leads to a pointer shift proportional to the weak value of
the measured observable. The effect is usually derived by considering a
perturbative expansion of the time-evolution operator corresponding to the
measurement. We show that the AAV effect can be instead derived given three
simple conditions on the pointer states. The most important condition is that
the pointer states be approximately indistinguishable. The other two conditions
relate the observables used to characterize the pointer with the measured
system observable. This intuitive approach does not require any model for the
measurement process, but is completely consistent with the usual approach based
on the von Neumann Hamiltonian. Moreover, this approach predicts and explains a
new phenomenon, called weak echo, where the usual weak value behaviour can
re-emerge at high interaction strengths.
