Exchange technique: blind versus visual
The 4x100 exchange is blind and fast. The incoming runner calls the handoff, and the outgoing runner is already at full speed, arm back, never looking around. The 4x400 exchange is visual and slower by necessity, since the incoming runner is often badly fatigued after a full lap, so the outgoing runner watches the baton arrive and takes it as a controlled handoff rather than a blind stick pass. That single difference shapes almost everything else about how each relay is practiced.

