A Fischer carbene can undergo nucleophilic abstraction where a methyl group is removed.  With the addition of a small abstracting agent, the abstracting agent would normally add to the carbene carbon.  In this case however, the steric bulk of the abstracting agent that is added causes the abstraction of the methyl group.  If the methyl group is replaced with ethyl, the reaction proceeds 70 times slower which is to be expected with a S2 displacement mechanism.