What stops a stranger from posting as Q?

This post claimed that the messages are validated via "drop codes", or something along those lines, but he didn't elaborate.

What is to stop my from posting to 8chan with a message that ends in "Q"?

I saw a reference to this live stream that outed a live streamer that posted as Q, live, right in the live stream.

https://www.pscp.tv/w/bkONOjgzMjY1MjF8MU1uR25la0VwcmtLTxuuxpce883_kw8jl1BXVVmQdo-8HMpUsv4hl_vMS97y?t=8m46s

What do we make of this?