Before revision:  One hypothesis proposes that rapid action potential onset is caused by the  separation of the axon initial segment, that is the initiation site of action potentials, from the soma, and that this spatial separation also results in ultrafast population encoding.
After revision:  One hypothesis proposes that rapid action potential onset is caused by the electrotonic separation of the axon initial segment, that is the initiation site of action potentials, from the soma, and that this spatial separation also results in ultrafast population encoding.