This skill is automatically checked when you cast a bardsong. Each round of casting time is checked up to half the casting time. If a successful check is made that round is removed and casting time reduced. Thus a person with 100% speedsong will have 1/2 casting time as they will succeed each time check up to half. A person with 50% speedsong will succeed only half the checks on average and result in a 1/4 reduction in casting time.
Note: This does not work on mental spells, and only works when using the lute.