It’s surprising how Blixen is comfortable enough with the idea of a glider in 1937 to use the concept so casually, since hang gliding and paragliding didn’t really get started until the 1950s. It turns out that gliding was a very popular sport in the 1930s, using a predecessor of the sailplane called a primary glider:
Primary gliders were generally launched by bungee cord, whereby a rubber rope was arranged in a “V” with the glider at the apex. The ends of the rope were pulled by hand to launch the glider from a slope.
Modern sailplanes (commonly launched by towing them behind a plane) can also be launched similarly, using a winch system.