Surfboard flex patterns.

So, what is a flex pattern and how does flex occur in surfboards when they are being ridden ?

Firstly with regard to the term 'flex pattern'. It is used by hundreds of surfboard shapers throughout the industry without any reference to what it might mean, and with no examples of what a flex pattern might look like, presumably in the hope ( as is usually the case) that someone 'higher up' in the hierarchy knows what it means and in the certain knowledge that obfuscation creates an impression of deep knowledge where there is none.

So in other words it is BS !
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What really goes on with surfboard flex?  In order to get a surfboard to flex one needs  downwards force and  upward force opposing it. The standard way of describing this effect is to show a surfboard supported in two places with a rider applying downwards force in between the two supports.

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Is this typical of what really occurs in the water ? 

The answer is ( in most cases ) 'No'.