Recently we brewed the first of two batches of our famous porter. Every year when we make this beer we like to try to get a second session beer out of the leftover sugars in the mash.
Generally to do a parti gyle beer you would use only the initial and strongest wort for the first beer sparging very little or not at all, the second beer you would rinse the sugars out of the grain (sparging) and get a second lower alcohol beer.
Honestly were more concerned with getting the first beer we send to the boil kettle reproduced the way we originally made it, our method is not exactly a true parti gyle but it does get us two beers out of one grist. We mash and sparge our first beer exactly as you would a non parti gyle beer with the exception that we stop the sparge a bit earlier than we normally would. Once the first beer is in the boil kettle we add some more grain to the mash tun (about 1/4 of the original malt bill) top off with some hot water and let the mash sit while the first beer is finished. Once the boil kettle is free we run off all the second wort add yet more water and run off again (a process called batch sparging), This keeps the gravity as high as possible.
For our second beer this time we were the least careful we have ever been regarding a beer, we didn't measure out any of the hops (nugget for bittering and goldings for flavor/ aroma) or pay any attention to the gravities.
We like this method because it allows us to produce the amount of beer we want and to keep our original recipe intact. The second beer is just for fun, it allows us to get some essentially free session beers and to experiment with hop varieties we may not have used before.