Hi there, with the size of kites you mention, a 50cm will not have enough leverage to turn the kite.You will usually require a bar about 70cm long. (These smaller bars are designed for use with kite surfing type kites, as the lines are attacted to the kite at the very end of the tip, therefore eliminating this leverage problem.)
Regarding the depower line, with RAM air kites, the fwd/power lines are attached to the bar ends,(giving you your left/right turn control) and the brake lines are attached to the ctr depower line, which in turn can be attached to your harness, or as flexi do with there new kites, to a safety reride system.Therefore, as you move the bar up and down, so you are applying/releasing the brake.
Hope that helps.