KNIIIO2 + K2MnVIO4 + H2O MnIVO2 + KNVO3 + 2 KOH

This is an oxidation-reduction (redox) reaction:

MnVI + 2 e- MnIV (reduction)

NIII - 2 e- NV (oxidation)

K2MnO4 is an oxidizing agent, KNO2 is a reducing agent.