I wondered about this for quite a while and eventually saw the answer while looking for other information. Sometimes I make a change to some startup files and I just want to rerun the startup without having to go through POST and all the other startup before the computer gets to the OS.
With root privileges execute the command:
shutdown now
This will give you a screen that looks like
Enter full pathname of shell or RETURN for /bin/sh:
Press enter to select the shell. I’m not qualified to say if this is the same as booting into single user mode or not.
Type
exit
The system will run the startup scripts and ask you to log in just as usual!