backboardd

backboardd is a daemon that runs alongside the SpringBoard daemon. It has been introduced in iOS 6, aiming to offload some of Springboard's responsibilities, chiefly that of event handling. Prior to its introduction, SpringBoard was effectively the UI event sink for iOS, as is WindowServer in OS X. With backboardd, all touch events are first processed by this daemon, then translated and relayed to the iOS application in the foreground (i.e. to its UIApplication event loop).

As a simple experiment, login over SSH to send a kill -STOP signal to the daemon, then touch your device. If you kill -CONT the daemon, you will see your touch events played out, as the events are consumed from I/O Kit by backboardd, and relayed to the foreground app.