Energy Management and proper coordination. Here is a similar maneuver from a different angle.

https://www.youtube.com/watch?v=PQqq_SDCwnA
(Skip to about 2:25 into the video)

https://www.youtube.com/watch?v=TVBIIHNt7Rg
(Double Pass)

Definitely not recommended for everyone, but with some practice this can be a very graceful and exciting demonstration. It is this kind of demonstration that can motivate younger audiences to become involved in soaring and bring new members out to soaring clubs. It is one of the few things we can do to make our sport exciting for others. When done properly this maneuver is safe.

-SW