To support self-restarting services, winsw exposes WINSW_EXECUTABLE environment variable into the forked process,
which refers to the full path of WinSW.exe that's managing the service.
To restart the service from within, execute %WINSW_EXECUTABLE% restart!.
Note that you are invoking restart! command, not restart command.
This hidden command is a flavor of the restart operation,
where winsw creates another winsw process in a separate process group,
and restarts the service from there.
This additional indirection is necessary because WinSW will kill child processes recursively when it stops a service. Windows Service Control Manager (SCM) doesn't provide the restart operation as an atomic operation either, so winsw implements restart by a sequence of stop and start. The second winsw process in a separate process group ensures that winsw can survive this massacre to execute the start call.