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.