If you've ever trained with me, you've heard me say over and over (other than lean forward) "SWING YOUR ARMS!" i have always been a fan of, and will every single time, tell you to use the full extension of your arm swing. swing both of your hands all the way
(comfortably) behind you. as you swing your arms forward- don't bend at the elbows, keep them straight (other than the very slight natural bend that comes from remaining relaxed), don't tense up, and then reach out fully in front of you. this works amazingly to carry you upward and forward. the pendulum effect will carry your upper body weight making the explosive push with your legs even more powerful. http://www.myphysicslab.com/pendulum1.html
with this information, i figured that if i had added weight at bottom of my pendulums, i'd get more force in the direction i want to go, so i got wrist weights-- just one pound on each hand (nearly 2% of my body weight). this maximizes the full extension of the arms and also keeps you from pulling back too soon; continuity in the swing is essential. in a regular broad jump, i was able to get inches farther. it was noticeably easier to achieve that extra distance. in a stair leap, for example at kezar stadium, i could exert a lot of effort (without weights) to jump to the fourth stair. with the weights, it was much easier.
i'm not saying to train or do parkour with weights on your wrists. they are training wheels and they are cheating. so, i'm only saying that they help you realize the importance of maximizing the power of a good arm swing-- laterally and vertically.