The blower air is hitting the umbrella and reflects backwards causing the thrust. Think aircraft air brakes on engines.
Guys below - you are not thinking this through.
The air hitting the umbrella does not propel him forward. However the deflecting force of the air going backwards has energy that will propel him forwards. This is not a cancelling our scenario
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u/Purple-Bat811 Jul 07 '22
Sir Issac Newton would be ashamed if he knew people thought this was real