CPU load is calculated once across each monitored interval as specified by the -f option. If the CPU load for a given process exceeds the threshold as specified by -c it is killed. The threshold can be ignored for a specific number of monitoring intervals by specifying the -i option. The process will only be killed if it exceeded the monitored threshold every time the CPU load was calculated across the ignored intervals.
For example: -f 30 -c 50 -i 5 means monitor every 30 seconds and when the CPU usage for the process is 50% across each of the 5 monitored intervals (ie 30 x 5 = 150 seconds) then kill it. Note that -i also works in conjunction with -m.