'Reverse ... a change to the opposite not only in side or position but also in direction, order, sequence, relation, or bearing.
An interesting definition that of course had me thinking of all the wild possibilities. This little mouse got lucky! In a reversal of fortune and role, the mighty mouse is getting the better of the timid tiger. Life can be strange sometimes so be ready to learn and break the norms.
Reverse psychology seems to have given the mouse a bit of courage. Someone must have told him "you can't scare a tiger, you little fellow!"