Ah, the Bullenbeisser (copbiters). They have some crazy vids, I've seen them do a burnout in the back of a van that was travveling on the Autobahn.
The flames are made in 2 ways:
- rev the shit out of your bike till the exhaust is glowing hot, not keep playing with the throttle. As soon as you let it go, the unburned fuel mixture will get into the hot exhaust en ignite
- again, rev it like mad, but in the meanwhile keep switching on and off the kill-switch. As you switch is back on, the unburned fuel mixture in the exhaust ignites
Needless to say I wouldn't buy a 2nd hand bike of their hands.