The old ones that had two to four D cell batteries in serial in them would often get so the connections between the batteries and the terminals in the flashflig

The old ones that had two to four D cell batteries in serial in them would often get so the connections between the batteries and the terminals in the flashflight casing would be corroded or misaligned. So you turn it on, the light doesn't come on, then you whack it a few times, and things line up and the circuit gets completed to turn on the bulb.