In the German television they had exactly this clip in a show, where they tried to proof, if the clip is real or fake.
They came to the conclusion that it will not work. The bike has to maintain a constant speed of min. 25 km/h to not to fall. A person is not able to keep up this speed especially if he fell down.
They had a professional racing driver in the show and a stunt man. Both tried to do the same thing. If they keep the bike on 25 km/h and just jump off and start running immediately, the stuntman was able once to catch the bike just before it crashed. But no hope if he falls off the bike.
The trick was very easy... (And that's what they did in the clip) They put the bike with the front wheel to a kind of trailer and pulled it by a car.
The car slows down and gave the driver the chance to keep up with the car/bike. Then when the driver was back on the bike they start accelerating again.
Akki.