Coil testing

[DavoVP]

This is quoted from another thread.

" Ok fellas, to test your coil packs,
With the engine off, take the first 2 leads of the first coil pack and then start the engine, now have a look @ the metal tips on the coil pack, the spark should be jumping from tip to tip, if the spark jumps anywhere else then the coil is stuff/on its way out! If the coil is ok then turn engine off, replace leads and move onto coil pack num2 and so forth on number 3! "

Would any one be able to tell me how and why this works ?

I had a coil that showed ok on the multimeter, But when i performed this test it arced out through the plastic caseing once every 2 or 3 seconds or so.

I replaced that coil and all my problems went away.

[DavoVP]

Any ideas ?