Like Jujitsu, Aikido is a martial art that needs an understanding of the physics of forces, torques, stability and rotational motion.
In the diagram above, figure 1 shows a simple technique in aikido if you are grabbed from behind on the wrists. You bring your hands up, (fig 2) slightly shifting your opponent off-balance, bringing their center of gravity up, and allowing their body to be manipulated more easily. You then crouch down, allowing your already-off-balance-opponent to be flung over your head.