Its called Decoction Mashing a it's type of mash in which at least one mash rest temperature is reached by removing part of the mash, boiling it in a separate vessel, and then mixing it back in to raise the temp of the mash.
Decoction Mash Procedure
The basic procedure for performing a decoction mash is very simple. Water is added to the grist to reach the initial mash temperature. Once the first temperature rest is complete, a portion of the grain and water is scooped out of the mash tun and into the kettle or another heated vessel, where it is brought to a boil. The portion removed, which can often be as much as a third of the grist, is called the decoction.
The decoction may require stirring during heating to avoid scorching the grain; this adds some extra work during the mash. The decoction step also adds time to the mash process, since a decoction cannot be heated as fast as infusion water and it is usually boiled for 5 – 45 min. After boiling, the decoction is returned to the mash tun to achieve the next temperature rest.
The easiest way however is to estimate the decoction volume with a simple formula like this:
decoction volume = total mash volume * (target temp - start temp) / (boil temp - start temp)
and add about 15 - 20%. The idea is to decoct more mash than necessary. When the decoction is added back to the main mash, it is not all added at once. Instead it is added in steps while the temperature of the mash is constantly checked. This requires a thorough mixing of the mash after each addition. Once the target temperature is reached the remaining decoction is left to cool and added once its temperature is close to the mash temperature. By doing so one can account for additional factors that effect the actually needed decoction volume such as: evaporation during the boil, unexpected temperature drop in the main mash and others.
Here is the article I got this from; http://braukaiser.com/wiki/index.php...oction_Mashing