Vw/Audi coil grounds

Vw/Audi, they on there Vr6 coil setup use both body and engine block to ground at the same time. I noticed your rule at the end of the session that never both.. how come they break it and what's the reason why not to do this.

On most of the VAG coils they have a ground for the primary winding (i.e the low voltage/high current side of the transformer) and a separate ground for the secondary side (high voltage side of the transformer).

The high voltage ground pin ideally needs to form the shortest loop for current to flow from the spark plug back to the coil - this would typically be the cylinder head. The low voltage ground ideally needs to form the shortest most direct path back to whatever the power source for the coil is - some may argue that is the alternator or battery - I guess in this case they consider chassis is the better choice for this.

But also be aware big auto manufacturers have many other considerations compared to a DIY guy doing a low volume harness. They may not always choose the best practice - it only needs to be good enough to do the job, they also need to consider things such as cost, ease of assembly, risk of long term failures & maintenance and probably many other things like RFI legislation and interference with Bluetooth, FM radio and all that other modern stuff.