How does the inband monitor work?

The inband monitor is a completely different kind of monitor from the other monitors offered by BIG-IP in that it doesn't send any content to a pool member itself (it's not whats termed an 'active monitor').

Instead, it looks at the success of connection setup for load balanced traffic - if a SYN is sent to a server and the server doesn't respond to with a SYN/ACK, that's an error and if the number of errors in a rolling time interval breaks a threshold, the server is marked down. The inband monitor will release the pool member from it's down status after a period of time (300 seconds by default), and begin tabulating errors again.