BIAB is a method that exposes the grist (crushed grain) to the full volume of water required in the brew at the start of the mash. Traditional methods such as batch-sparging and fly-sparging expose about half this volume to the grist at the beginning of the mash. After the mash this is drained into the kettle and then the second half is introduced to the grain, sometimes in stages, and subsequently drained into the kettle. To do this requires two extra vessels, the 'hot liquor (water) tank' and a 'mash tun.' BIAB enables the kettle to also act as the HLT and mash tun.
Sparging really means to "rinse" the grain. BIAB does this. It simply starts the rinse earlier than traditional methods.
So, what is the "No-Sparge" Method?
The true 'no-sparge' method involves not allowing about half of your full-volume to ever come into contact with the grist. So, the grist does not get 'rinsed.' Instead, the water usually used for sparging gets added directly to the kettle.
Obviously, a failure to rinse means that a lower original gravity will result. To combat this, 'no-spargers,' will increase their grain bill by about a third.
If you need a third more grain, why would anyone "No-Sparge?"
In the BJCP manual there is an article by Louis Bonham. The article begins with...
A few years ago I asked George Fix and Paul Fairnsworth what techniques I should use to make the finest possible all-grain beer. Both of them immediately told me, "Don't sparge.
The theory is that it creates a richer, maltier beer. For traditional brewers it also speeds the brewing process up by eliminating the sparge step. Most brewer's palates though would not be able to detect the difference between a no-sparged brew, a full-volume brew (such as BIAB) and a traditionally sparged brew.
Bonham designed an elegant and thorough experiment between a 'no-sparge' beer and a 'sparged' beer. Eight out of thirteen judges/industry professionals were able to taste a difference in 'triangular' tastings. The findings of these eight gets a little vague... "most of [these eight] reported the ['no-sparge'] beer tasted richer and maltier."
How Would I "No-Sparge" a BIAB?
I personally would not advise any brewer to test no-sparge unless they simultaneously brewed a control which is impossible unless you have two BIAB set-ups*. Expectations can have a great effect on taste buds .
Some brewers however seem to have a knack for identifying small differences between beers of similiar recipe brewed on different systems at different times. These brewers can easily do a 'no-sparge' BIAB. Simply take your normal recipe and increase the grain bill by a third. Instead of adding all your water to the mash, hold back a half. Mash as normal and pull the bag. Add your remaing half of water before the boil is reached. (Preferably pre-heat this to at least 76 C. If this is not possible, just add it gradually pre-boil.)
* I'll write how to do this if it is of interest.