PURPORT

Lord Krsna promised not to fight in the battle of Kuruksetra or even take up a weapon. But when Bhisma wanted to keep his own promise to break the promise of the Lord, the Lord immediately got down from the chariot, and to make Bhisma's promise true He picked up a chariot wheel and rushed forward to kill him. This is a quotation from Srimad-Bhagavatam (1.9.37).