http://www.willswing.com/pdf/pdf_man.asp?theModel=T2

Batten tension

The outboard most batten on each wing is tensioned by looping the batten string
over the notched end of the batten twice. The remaining battens are all fitted
with lever tips. The outboard batten with strings should be adjusted quite tight
for maximum sail cleanliness.

On the battens fitted with lever tips, it is important to adjust the lever tips
to set the right batten tension. The lever action makes it easy to have the
battens set too tight. Batten tension is best judged by noting at what point the
batten begins to tension the sail as you pivot the lever tip down into its
locked position. This is the point at which chordwise slackness begins to be
pulled out of the sail, the high point of the batten begins to make a prominent
ridge in the top surface, and a noticeable increase in resistance is felt in
pivoting the tip downwards.

On any battens inboard of the crossbar junction, this point should not come
until the lever tip is within no more than the last 20 to 30 degrees of travel
before becoming straight. On battens outboard of the crossbar junction, this
point should come when the lever is between 30 and 40 degrees of straight. To
lengthen the batten, simply unscrew the batten tip shank from the batten, to
shorten the batten screw the tip further in. Each three turns changes the length
of the batten by 1/8 of an inch.