If you hold any kitesurfing boards horizontally in front of you (side profile view) the rocker is the line of curve you will see running from tip to tip of the board.
Rocker is included in kitesurfing boards to assist with:
* a smoother ride
Rocker helps with turning because it allows you to move your centre of gravity around on the kitesurfing board, move your centre of gravity to the back of the board and the front of the board will rise out of the water, this decreases the wetted area and means that when you want to turn the pivot point of the board is further back (due to there being less board in the water at the front) and thus the board turns more easily and quickly.
Rocker allows kitesurfing boards to naturally tilt back and forth when riding in waves. A board with no rocker (ie a flat board) will tend to ride straight over the wave and then slap down on the back of the wave wave when the front of the board becomes heavier than the back of the board. A board with rocker will roll back on its rocker as it heads ups the wave and then rock back down onto the front of the board as it comes down the wave thus cushioning the impact and giving a smoother ride.
So although a board with rocker will turn easier because of its rocker it may be harder to keep going in a straight line, this is because the water will cling to the shape of the rocker profile and naturally start to turn the board in that direction, the board is also much more likely to suddenly lose grip because of the shape of the board in the water.
In a nutshell (and simplifying greatly!) rocker will help your board to turn and give you a smoother ride but too much rocker will make your board difficult to ride in a straight line and give you less grip in the water.
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