Think of a capacitor as either a battery (DC Electron Storage) or as a filter (AC Signal Filtering). In line (Series) connection it blocks DC but allows AC to pass through. The ground side of the HEI is what fires the spark in simple terms. There's actually a whole bunch more going on in several processes.
The signal from HEI in simple terms is alternating current. In actuality it's more of a square wave, but that has to pass through so the tach can read it. The capacitor also helps filter stray and large spikes that make the needle jump around.
Basically (Simple Terms) the capacitor is designed to help regulate (filter) the signal from distributor to tach, which thereby makes the displayed measurement more accurate.
In car audio a capacitor is used in line between amp and speakers as a high pass filter to block out unwanted frequencies that translate to dc in the speaker(conductor). Different mF ratings for different frequency cut offs at -6db per octave iirc. People called them bass blockers. If you've got one use it, I've fried cheap tachs before on MSD setups not using them.
GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.