I 'rinse' sparge, which consists of dumping the biab bag in my spare fv along with 5l of water and give the bag a good wiggle with my plastic paddle, then leave it to soak till the remaining wort is about to boil
Pull the bag out and give it a good squeeze and pour the 5l of sparge water + whatever I squeezed out of the bag in to the kettle and away we go
No idea what effiency I get from doing it this way, over the much more complicated sparge methods, but it's simple and works for me.
I might take a sample next time I brew and take a gravity reading to see what I get, but I usually achieve a 70% efficiency, which is more than acceptable for biab