The CPU can react to very short pulse of digital IO. The detection of IO state changes is a silicon-level hardware feature so detection is in the nanosecond range. Such a short pulse will cause the stream value to change state rapidly twice and if you are outputting on something like CAN then it might not ever appear on the bus. This "hold positive" option will force any (short-term) high pulse to be held in the data stream for the number of milliseconds specified.