I know it sounds strange and found it hard to believe myself.
This is info i got from some of my smog classes.

Standard coil spark starts in coil goes through plug wire ,jumps plug gap,into frame and back to battery.(simple explanation)
Waste spark ignition goes coil,plug wire,across gap through engine block,jumps gap backwards(?)on the other plug in its pair ,through plug ,then wire on back to coil.never goes back to battery.The spark can start at either plug and and go in any direction.This is why its called a/c current,and waste spark.
Was hard to understand and may not have explained it as well as the book,but thats what we where taught.
So closed circuit,fires both plugs,but not at same time and travels in either direction.
Hope did not confuse,class was 2 or 4 years ago
Rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car