This graphic was made for one of the first landing attempts, and it shows an unusual scenario: a landing on the droneship where the ship is positioned much closer to the coast than on later missions.
For a normal droneship landing, the stage does two burns: reentry and landing. Its trajectory is roughly a parabola, and doesn't exhibit this extra peak.
For this attempt, they did 3 burns: boostback, reentry and landing. The boostback burn has to kill all forward velocity, then accelerate the rocket on a parabolic trajectory towards the landing site. They did this to test the boostback burn without having the stage land on Cape Canaveral (which was considered too risky for the early landing attempts).
During the launch, the rocket is powered through the entire trajectory. At stage separation, the stage moves at 5000 km/h.
For the return flight to the landing site, the trajectory is an unpowered ballistic trajectory. The boostback burn has to provide enough energy to 'throw' the stage all the way back to the landing site. There are 2 ways to do this:
- accelerate the stage to 5000 km/h. This takes a lot of energy.
- accelerate the stage to a lower speed, and provide some energy by boosting it to a higher altitude. The width of a ballistic trajectory is determined by the speed of the rocket and its altitude. This takes less energy.
The graphic is also usable for landings back at Cape Canaveral.