  The decay of $\Lambda$-hypernuclei without pion emission, known as Non
Mesonic Weak Decay (NMWD), gives an effective tool to investigate $\Delta$S=1
four-baryon interactions. It was theoretically suggested that the two-nucleon
induced mechanism could play a substantial role in reproducing the observed
NMWD decay rates and nucleon spectra, but at present no direct evidence of such
a mechanism has been obtained. The FINUDA experiment, exploiting the
possibility to detect both charged and neutral particles coming from the
hypernucleus decay, has allowed us to deduce the relative weight of the two
nucleon induced decay rate to the total NMWD rate. The value of
$\Gamma_{2N}$/$\Gamma_{NMWD}$=0.24$\pm$${0.03_{stat}}^{+0.03_{sys}}_{{-{0.02_{sys}}}}$
has been deduced, with an error reduced by a factor more than two compared with
the previous assessment.
