When a squirrel tries to eat from the feeder his weight closes off the feeding ports to deny him access to the food. You can also adjust the tension to make it sensitive enough to shut down on the large starlings or blackbirds. Backyard birds average 0.3 – 2.0 ounces while blackbirds are about 4 ounces and squirrels are about a pound.
The video of the squirrel performing acrobatic acts is very intriguing but most squirrels turn tail and run after their first encounter. I tell people there is always hope that in the spring a new potential "flipper" could be born.