I'm sorry, there is no manual for download.
The "Respin period" feature is useful only for some systems which don't properly 'listen' to a fan speed command. There are some systems, which when told to set a certain fan speed RPM don't hold the target speed, but after a while their own fan speed control kicks in. So in such cases the user can specify a frequency (/period) at while HWiNFO checks whether the fan is really running at the desired speed and if it doesn't it sends the desired fan speed command again.
The "Respin period" works only in Custom Auto setting.
If you need a fixed speed you can try the Manual setting, but this is not retained after reset. For such mode you will need to configure the Custom Auto setting so that it will always set just one fixed speed (i.e. define one speed for any temperature).