Within the sidechain. Not main.
Notice the single quotes on either side of that word "signed" ? In the github version they are solid double quotes. Meaning, something like or similar to.
Bip300 does not sign a transaction the way you or several people use your keys to sign a transaction.
It's a vote over time ON THE SIDECHAIN, that some txn is valid. It's like saying changing from single round voting to 3 rounds of voting in a small town election, changes the outcome of national presidential election.
Bip300 ads one new Op code, with sidechain parameters and no mainchain affects for people not participating in it. It does not increase the number of bitcoins, it does not change the turing incompleteness of bitcoin script or break anything on chain.
It's going to happen, if you don't get it at this point i've spent enough time explaining, and wish you the best.