I lift the bag a little and fire up the burner. Then I give a quick stir under the bag, lift it, let it hang and squeeze. I'm just trying to speed up time to boil.
I think mashing out helps in rinsing sugars with a traditional water to grist ratio. Also BIAB your not sparging for 45 mins or so like a fly sparge so I've never found the need to "lock" in the fermentability of the wort. Some BIABers mash with less water and make up the rest of the pre boil volume when they rinse grains with higher temp water.