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 whoever runs this page cannot know the result in advance, however far ahead the draw is scheduled. The miner who finds the last of those blocks sees it first and could discard it; here is how much that can matter.
Want to use the beacon in your own code instead of this form? See the JSON API.