Just imagine yourself holding the clew in your hand and trying to pull it one direction or the other to make the foot tighter or the leech tighter. Fluttery means loose, so you'd pull down more if the leech is fluttering, back more if the foot is fluttering.
Now position the jib car so that the sheet is pulling in exactly the direction you determine. Moving the car forward pulls down on the clew, moving aft bulls back on the clew.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27