A live demonstration of getrandombeacon: a randomness source derived from real GPU mining work, not a server-side random number.
Starting a draw locks in the current block height. If you pick a draw time, nothing about the eventual result is decided in advance, this server just waits in real time until that moment, then looks at whatever the actual chain height is right then and uses a few blocks after that. The winner is decided by mixing together the mix_hash values of blocks that don't exist yet at trigger time, so nobody, including whoever runs this page, can know the result in advance, no matter how far ahead the draw is scheduled.