This manouvre was invented by the Rodriguez brothers, Félix and Raul, in 2001.
Two pilots start to make big dolphines (pitches) next to each other and for opposite rhythm. While one goes up, the other one goes down and so on. Once there’s enough energy and height difference between the two pilots, they center themselves and continue the movement in line so they pass under/over each other. Of course, there’s a high risk of crashing, if the timing goes off.