[dhotson]

It's because the server has to create a tarball on demand, which can take a while.

So, instead of having your web request unresponsive for a minute while it's processing, they queue up a background task on the server and give you a "hardcore archiving in process.." response immediately.

Then I imagine they do some ajax polling to check when the background task has finished and the archive is ready.

It's a really good technique if you want your web app to be responsive. We do it at work for basically anything that will take more than a second.