Slack
Removing slack is the quest of all fly fishers be it single- or double-handed dudes and dudettes. Books, DVD, Videos Clinics, Clave and the list goes on are all pointed at removing slack.

Now my feeble attempt to shorten the long subject: "Tempo and Timing" with the correct "applied effort" within the "casting motion" is the goal to remove slack. Tempo= smooth acceleration; Timing= when to start the next element of the casting stroke; applied effort= the energy transferred to the rod in each element of the casting stroke; and, casting stroke= the entire motion of co-ordinated body movements (arms, hands and body) to deliver energy to the rod. Klem