The story
The phrase became popular in the Second Industrial Revolution, when sometimes vacuum tubes or soldered connections would become loose.
Etymology from Wiktionary, CC BY-SA 4.0.
technical tap
Today it means: A tap or a slap on a device in a way to either make it work correctly again, or break it completely. For example, hitting a TV to get its reception back, or hitting a jukebox back to life. Full definition →