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It's a social media campaign where people allow the creator of the Thunderclap - https://www.thunderclap.it/ to post something on their social media channel.

For instance, you would create a thunderclap to promote your Youtube Steemit video, then advertise the thunderclap here on Steemit. People join with their social media account and then at the predetermined time, the same message is disseminated across the web over many social media accounts. Hopefully, resulting in a sudden influx of interest.

You'll need a lot of views in a short amount of time to get on the trending page in YouTube. So coincide the thunderclap with the release of the video. Although this might all be too late now. But at least you know for next time. :-D

Check out this recent example - https://steemit.com/marketing/@cryptoctopus/it-s-time-to-thunderclap-not-satisfied-until-we-get-1000-people-to-participate

And Good Luck :-D