Besides corn sugar and DME, beer can also be carbonated with unfermented or actively fermenting wort. This procedure is usually used by breweries that bottle ferment their beers. And the reason for this is simple: freshly fermenting wort is readily available in a brewery. It also has several advantages over the priming corn sugar/DME:
- the flavor of your beer will not be changed, since you are adding part of the original wort (same IBUs, same malt profile, etc.)
- the apparent OG will not be changed. (If you use a highly concentrated sugar solution your FG and ABV will be as if the beer had a 2-3 GU higher OG.)
- fresh and healthy yeast is added which does a better job in scrubbing the beer of O2 and off-flavors. In this case you can also use your bottles as a yeast bank since it is not the least flocculant yeast that is at the bottom of those bottles.
- carbonation times will also be more predictable if you add fresh yeast.
- you will have more beer to bottle and can get 5gal out of using a 5 gal carboy for fermentation.
The main disadvantage is the added complexity.
If you want to prime with Speise (aka. gyle) just keep some of the original wort from the brew day. Some brewers keep it in a jar in the fridge, but I like to store mine in the freezer. This way I don't have to worry about something growing in the wort for the next 3 weeks that it takes until I bottle. I also don't worry about sanitation when I collect the gyle. As shown in the whirlpool how-to, I get most of it by filtering the hot break that is left in the kettle through a paper towel. This takes some time.
Once I need it, I will take the gyle from the freezer, add a little water to compensate for the boil-off and boil it for 10-15 min. This will get it sanitized and I can use it after it has been chilled (either overnight or in an ice bath). You don't have to chill it if you plan to bottle without additional yeast.
 How much do I need?
The amount of gyle needed for a batch can be calculated with this formula:
Quarts of gyle = (12 x gallons of wort) / (specific gravity - 1)(1000) (Source)
I requested that a more precise calculation of this (one that accounts for attenuation and already existing CO2 in the beer) be added to BeerSmith.
If you don't add new yeast, the carbonation time should be similar to the use of DME for priming.
In order to bottle with Kraeusen you need to do a little more. Prepare a starter with the calculated amount of gyle plus another pint of gyle. Aerate it and pitch this starter with the yeast you want to use for bottling. There are some advantages of actually using a different yeast that you fermented with. I have heard that some brewers actually bottle lagers with ale yeast. Or you want to choose a very flocculant strain. According to White Labs, no significant amount of flavor will be produced by the yeast used for bottling and ale yeasts work faster too. I usually harvest yeast from the primary and will use the same yeast for bottling.
You want to be ready to bottle when the starter is actively fermenting (at high Kraeusen) but hasn't fermented lots of the sugars yet. The CO2 should end up in the bottle and not in the atmosphere. Once you are ready to bottle, you want to add the desired amount of that starter (see formula above) to your bottling bucket. If you think it has been fermenting for a while already, you can add a little more. Here is the tricky part. You don't really want to get the yeast sediment that is at the bottom of the starter, only the healthy yeast that is currently in suspension is necessary. That's why I suggested adding another pint of gyle when making the starter. Leave that in there and try to pour only once, as tilting it back up will get a lot of that yeast in suspension.
Now add your beer, stir, and bottle as usual. Carbonation will be complete in 5-7 days. From my experience there will not be more yeast sediment than using conventional priming methods. But I also keep my secondaries cold (~50°F for ales) which causes more of the original yeast to settle out.
The same method is used for carbonating my lagers in a corny keg before I start lagering. But there I just make sure that enough Speise is added to get above the desired CO2 level. I then monitor the pressure build-up and blow-off any excess CO2. This has the advantage of adding fresh and healthy yeast before the lagering is done. Finally I rack it off all the yeast into a serving keg.
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