Slapping the remote is not a sure-shot way of making it work.
But the “technique” does work sometimes, and here is the science behind it.
Because batteries sit in remote control for months or even years, an oxidative layer forms between the battery’s negative terminal and the terminal (spring) of the remote.
Forming of the oxidative layer, i.e., oxidation, is the same process that leads to browning of the apple or rusting of an iron bar.
The oxidative layer has the characteristic that it blocks the flow of current.
Now, if the remote control used a lot of power, it would easily overcome this resistivity posed by the oxidative layer.
But remote controls use small amounts of power, and so, an oxidative layer is enough to hamper its function.
When we slap the remote, the batteries inside move slightly to disturb the oxidative layer.
Small power (point 7 above) also explains why slapping is more often required in the case of old batteries (even less power).
As the resistivity weakens, the remote control starts to work again.
