Replacement of methyl groups of camphor is a reduction mechanism. Camphor is a bridged bi-cyclic compound. If you observe the structure of camphor the methyl group is placed with one carbon bridge (7, 7) and two carbon bridge (1). Attack from face of one carbon bridge is termed as exo attack whereas from face of two carbon bridge is termed as endo attack. So replacement will lead to mix of both and formation of two stereo isomers.
Reduction mechanism is often shown with a U-shaped arrow pointing the attack by ion such as in nucleophilic addition reaction.
