PostgreSQL kill - Sighup pid To reload the configuration files, we send the SIGHUP signal to the
  postmaster, which then passes that on to all connected backends.
  That's why some people call reloading the server "sigh-up-ing".
  
  As reloading the configuration file is achieved by sending the SIGHUP
  signal, we can reload the configuration file just for a single
  backend using the kill command.  
  
  First, find out the pid of the backend using pg_stat_activity. Then,
  from the OS prompt, issue the following:

kill  -SIGHUP pid



I dont understand bold words. Because we have many PIDs for backend  and  if we kill a PID, how can it get changes from reload configuration file (postgresql.conf)?
Probably this will help: 


  The Linux operating system normally terminates, or kills, a task by
  sending it a SIGHUP command. This stands for "Signal Hangup" and
  terminates a process on the system. If a programmer has not created a
  program restart command, or if you are having difficulties with an
  unresponsive program and wish to restart it, sending it a SIGHUP
  command will terminate the program and cause it to automatically
  restart.