Quoted from documentation

The '-st' and '-sf' command line options are used to inform previously running processes that a configuration is being reloaded. They will receive the SIGTTOU signal to ask them to temporarily stop listening to the ports so that the new process can grab them. If anything wrong happens, the new process will send them a SIGTTIN to tell them to re-listen to the ports and continue their normal work.

Otherwise, it will either ask them

  • to finish (-sf) their work then softly exit,
  • or immediately terminate (-st), breaking existing sessions.

A typical use of this allows a configuration reload without service interruption

 haproxy -p /var/run/haproxy.pid -sf $(cat /var/run/haproxy.pid)