Minecraft wiki entry for Sprinting, associated text: Sprinting and jumping while under a 2-block high ceiling causes a player to move at a speed almost twice as fast as regular sprinting speed, but it consumes a hunger point every second. It is also possible to jump 5 blocks when jumping out of the 2-block high ceiling.
This can be sped up even more by decreasing the ceiling height using trapdoors or by sprinting on ice.
A similar effect occurs if the player sprints and rapidly hits jump at the same time on a set of stairs.
Sprint-jumping actually costs less exhaustion per block travelled compared to pure sprinting, despite being faster. This is because exhaustion increases while the player is on the ground. While sprint-jumping, most of of the distance is covered in mid-air.