Mark is correct mostly. It works like this:
Intent : for use in 10 minute moves it reinstates the power of artillery but still allows freedom of movement melee and fire in any particular order. So the flow of your turn is not undermined by remembering to stop and do melee everywhere. You just need to remember to respect your opponents guns and zoc setups to defend them.
The rule: No unlimbered artillery battery and any other units in the same hex may be meleed if another unit formed or unformed has to be melee'd out the way to reach it.
The exceptions:
You can shoot a unit out of the way (by overkill)
Pursuing cavalry can ignore the rule if they are in the process of pursuing

(but not assisted by fresh units)
skirmishers, officers and supply wagons can be blitzed out of the way.
The rule does not apply to limbered artillery which can be meleed anytime
This rule is easy and allows a low house rules game with a bit more realism. To many house rules can be tiresome in a big engagement say for example Leipzig. Although as Andy Moss pointed out there is so much artillery in these later titles the rule may not be a good idea.
These things said I personally still prefer embedded melee in 15min turns.
Anyhow its been tested in a few games now and works fine from what we can see.