These are small device like nano arduino with a WIFI Module With this codehttps://github.com/spacehuhn/esp8266_deauther you can disconnect everyone you want on a network !
This is a cool method - However, there are apps which utilise the wifi chips in android phones to run the same type of attack - no need for the extra hardware
I'm not sure how effective these are but you can run something more effective or at least effective as the nano method using a standard laptop and a powerful wifi dongle / adaptor