Oh yes, a well made video would be really helpful here, but I don't recall if anything like that currently exist.
I will try again. When you are logged in on Steemit site, it means that you've used at least your private posting key. Browser holds it (this key is not sent to the server, it never leaves your computer, it's private after all) and uses it to sign transactions. So when you upvote something on the site, there's a transaction created on your computer, signed by your key and sent to the steemit servers, they are broadcasting it to the network, and the witness, who is about to create a block, collects all such transactions that are not yet included in blocks (up to the max size of a block) and verifies them and signs with the witness signing key. Such block is added to the blockchain permanently (assuming, that majority of other witnesses agree with its content, i.e. add their blocks on top of it)
So your signing is needed so the blockchain "knows" that it's you who are willing to send 1 STEEM to someone, but then signature of the witnesses is needed so the blockchain "knows" that such transaction is valid (i.e. that you actually had at least 1 STEEM etc.)
@gtg apprecaite your comment a lot. howeer i have to say the same as to colleague below, ill try to 'deal' with what you;ve put on weekend as when i have time for steemit during the week, my mind is not accepting this type of stuff! unfortunately :D but thanks, ill come back to it and then reply!