  In heavy ion reactions at energies around 1-2 AGeV the measured $K^-$ yields
appear rather high as compared to pp collisions as shown by the KaoS
collaboration. Employing IQMD simulations, we show that this is caused by the
fact that the dominant production channel is not $BB\to BBK^+K^-$ but the
mesonic $\Lambda(\Sigma) \pi \to K^-B$ reaction. Because the $\Lambda $
($\Sigma$) stem from the reaction $BB \to \Lambda(\Sigma) K^+ B$, the $K^+$ and
the $K^-$ yield are strongly correlated, i.e. the $K^-/K^+$ ratio occurs to be
nearly independent of the impact parameter as found experimentally. $K^-$ are
continuously produced but also very quickly reabsorbed leading to an almost
identical rate for production and reabsorption. The final $K^-$ yield is
strongly influenced by the $K^+N$ (due to their production via the
$\Lambda(\Sigma)$) but very little by the $K^-$N potential.
