Fast flight is a synthetic fibre, used for bowstrings.
It should not be used on any bow designed before the introduction of fast flight, because it has very different characterist elasticity than Dacron or natural fibers. Flemish twist or Flemish splice bowstrings are the traditional longbow string, with the loops made in the same manner as a rope by twisting and splicing, rather than being a continuous strand of string material with the loops formed by serving. A flat bow is a bow with rectangular limb cross section and the limbs tapered evenly from grip to nock. Examples of these, and examples of the D section style, have been found in neolithic strata in Scandinavian peat bogs. Here's one for you history buffs.