Swimming hand paddles increase resistance and build upper body strength and power, while also improving pull technique. A paddle's size and shape affect both the resistance created by the paddle and which aspects of technique (and strokes) the paddle helps guide.
Flat paddles (Like strokemakers and Arena Elite paddles) are primarily for building strength. Contoured paddles (such as the Speedo IM, FINIS Agility, and Manta paddle) help develop technique. Forearm fulcrums correct freestyle hand, forearm, and elbow positioning.