This is an attempt to make Shairport Sync interwork better with other audio programs. If you send a SIGUSR2 to Shairport Sync, it will disconnect from and release the ALSA device it's using, allowing other audio programs to take the device. Shairport Sync will continue to work normally is all other respects. Send it another SIGUSR2 and it will try to regain the ALSA device, but won't complain if it can't. Thus, to be exact, a SIGUSR2 signal toggles its connection with the ALSA device.
You can send a SIGUSR2 signal to a properly-launched shairport-sync daemon by executing shairport-sync -P (you might need to have root privileges). This will launch another copy of shairport-sync which will send the SIGUSR2 signal to the daemon process and then exit.
This is very experimental.