He was flying a glider owned by during an exhibition show, which was lifted by balloon and then released.
It is performed by doing a combination of a roll and a loop.
The example shows the symbol for a 270° turn.
Such a roll could be done with the aircraft following a nearly linear trajectory.
After the roll is completed the nose is usually 20 - 30 degrees below the horizon.
So you get back into your initial mount by doing a forward Barrel Roll.