There is a persistent myth about the function of a sword's fuller that it 'releases the vacuum' when the blade is thrust into a person. Basically, the idea is that the so-called 'blood groove' is there to aid withdrawing the blade from a person (or animal). In this scenario, it is said that the person’s muscles contract around the blade, which causes a vacuum, making the blade difficult to remove. With a 'blood groove', however, blood runs through the groove breaking the suction so the blade can be easily withdrawn.
There's only one problem, there's no evidence that this suction ever really happens. Those who have examined the theory repeatedly report there is no difference whatsoever in the difficulty of withdrawing a blade with a 'blood groove' versus one without. The theory has been tested and found wanting - if the blade can cut its way in, it can just as easily cut its way out, with or without a 'blood groove'. Since there is no blood-channelling function, the term "blood groove" is misleading and the more accurate term 'fuller' ought to be used instead.
On knives or swords less than 2 feet long, the fuller is mostly decorative. Only as a blade gets longer does the fuller have an increasingly important role as it both stiffens and lightens the blade. So, with a fuller:
• The blade is lightened as less material is used. The act of forging in the fuller actually widens the blade so once again less material is needed to create the required blade width. In stock removal (grinding) the blade would also be lightened as metal is removed. When combined with proper distal tapers, proper heat treating and tempering, a fullered blade will be anywhere from 20% to 35% lighter than a non-fullered blade without any sacrifice of strength or blade integrity.
• The blade is stiffened. An unfullered blade typically has only a 'single' centre spine. This is especially true in terms of the flattened diamond cross section common to most unfullered double-edged blades. However, a blade close to or exceeding three feet in length becomes rather 'whippy'. Fullering produces two 'spines' on the blade, one on each side of the fuller where the edge bevels come in contact with the fuller. This has the effect of stiffening the blade, and the difference between a non-fullered blade and a fullered one is quite remarkable.
From an engineering perspective fullers are a sophisticated forging technique. It was probably the fullered sword blade that pointed the way to modern 'I' beam construction. So, fullers were not 'blood grooves', or there to 'break the suction', or for some other grisly purpose. Rather boringly they simply serve an important structural function.