If you are doing some tests which require reboots (e. g. startup skripts, kernel module parameters, ...), this is very time intensive, if you have got a hardware with a long pre-boot phase due to hardware checks.

At this time, kexec can help, which only restarts the kernel with all related stuff.

First the kernel to be started is loaded, then kexec -e jumps up to start it.

Is as hard as a reboot -f, but several times faster (e. g. 1 Minute instead of 12 on some servers here).