Normally, the acid helps the whipped eggs whites hold their structure and is mixed in when the whites are still liquid.
In Pavlovas, many recipes state to fold in the acid AFTER the whites have been beaten to the stiff peak stage. JoyOfBaking.com says the acid helps create the marshmallow like texture inside the meringue cake.
I tried a Pavlova with the acid mixed in at the beginning like I usually do and got a marshmallow texture to the cake anyway.
Does it matter if the acid is mixed in at the start or folded in after the stiff peaks? If folded in, the acid won't be as evenly distributed.