The description from one of the other slingshot rocket launcher vids is:

Launching rockets in Germany requires all kinds of permits and licenses nobody ever gets. But for just 48 hours each year, the rules change. During these two days, every adult person is allowed to blast away with rockets and fire crackers!

The Slingshot Channel MUST take advantage of this once-in-a-year time slot. This time, we built a 2,5 meters high rubber based launch tower, capable of launching an unlit rocket about 50 meters up in the air! Add these 50 meters to the 60-80 meters that the rockets achieve by means of their powder charge, and you get some serious total height.

Of course many things can go wrong. The flimsy wooden shafts are not made to endure the stress of 50 kilogramms (110 lb) of a draw force applied by the rubber bands, and can break right in the barrel of the launcher. Also, if the timing of the fuse is bad and the charge ignites when the nose of the rocket is already pointed downwards, then the rocket will be propelled downwards and explose at ground level - effectively a surface to surface missile.

The Slingshot Channel tests all this... come and see the results!

It seems to be the season for it, people here are still letting off illegal fireworks 24 hours after New Years Eve (and they started several weeks before Christmas!).

That video also explains what's going on, it was his test run during the day: