Knut Bergqvist (Bergkvist) developed the graal technique in 1916 at Orrefors. He did take his inspiration from the French art nouveau glass of Daum and Gallé from this time frame. The name graal was inspired by the saga of the Holy Graal which contains the blood of Christ. Hence, the first graal pieces were ruby red and had the aspect of a red liquid freely flowing in the vase, which brings us to the analogy to the blood of Christ and the name Graal.
In the graal technique a colored layer of glass is encased by a transparent layer of glass. The glass is then allowed to cool down and when it is cold the design is applied by engraving or etching. The obtained “stock” of “blank” piece of glass is then carefully reheated and blown into its final shape. During this process the sharp edges of the design become smooth and the motive becomes more soft and blurred, giving it its typical Graal aspect. The technique can be expanded to use multiple colors. This combination of glass blowing and glass engraving became a huge commercial success for Orrefors and was the beginning of what can is nowadays seen as the Swedish art glass movement.
Here is a link to the Orrefors online glass museum showing a variety of pieces done with this technique.
Below you find an excellent tutorial in two parts providing a great overview on the technique.