People sometimes demonstrate how tapping a spot on their gear causes a
problem to start or stop. They wonder, "Won't this repair be
simple since I've showed you where the problem is?"
First of all, "simple" is a word you shouldn't use near a
repair technician. This is the "So Easy A Caveman Can Do It" faux pas.
Consider the executive desk toy called
Newton's Cradle. One steel ball swings down and hits a row of other balls.
As a result a distant ball moves, even though all the in-between balls stay put.
In the same way, when you tap on an electronics chassis, every distant circuit
board, electrical part, and wire gets the message. Intermittent problems can be microscopic,
or even quantum mechanical,
and are often
the most difficult to track down, even when tapping on the chassis with a feather touch.
In addition, mundane difficulties occur during any job. Stripped or rusted
hardware, tricky sub-assembly removal, and bad design or engineering can all take additional caveman-hours.