Why does hitting a flashlight when the batteries are low keep it working for a short time?

If a flashlight is getting dim or having trouble turning on due to low power in the battery, it's common to bang on the flashlight a few times and it will start working. The same goes for batteries in a TV remote, except for banging on the batteries, just running your finger over them and making them roll in their slots back and forth seem to make the batteries start working again.

Corrosion on the battery contacts creates resistance that reduces the current delivered. Vibration can wipe off some of the corrosion.

Also, the higher the voltage, the more resistance it can overcome to produce current. When a battery is low (at a 20% state of charge) it's voltage is lower than average, so the smallest amount of resistance that is in the system can temporarily start cutting the current.

Filament bulbs will just get dimmer and dimmer, but an LED requires a minimum voltage to operate, so when the batteries hit low voltage due to higher resistance in the system (usually from corrosion), they just shut off.