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how does this upvotebank work. does it take my password?

No

https://steemit.com/steemit/@upvotebank/gzsr5aw6

TL;DR just delegates unused voting power between users - never give away any private keys, and you'll be good ;)

But the upvotebank demands for password/private key before one can register right?

That might actually be true, I don't quite remember...

IF it asks for any private key, it should be your private posting key, which is "ok" as long as they don't ask for your main private key... but it's been a while since I signed ud - I'd read their project description tbh... :)