Okay I think (think) what it is trying to say is this.
If you wire to the index array function from another function, then it will take one clock cycle. For example if you wire the output of an add function to the index terminal, then the compiler cannot know what the result of the add is until it is ran. So it takes one clock cycle.
If you wire a constnat to the index array function the compiler can know exactly what you want, and do it in no clock cycles.
If you don't wire anything to the index array function, it behaves like you wired a constant of 0, and because it can know exactly what you want, it can do it in no clock cycles.
As the others have said, nothing wired is the same as a constant for the index. What the help is saying is you will use a clock cycle (and gates) if you wire in a variable of some sort to the Index.
Thank you for your answers! I agree that it should be as you say, so I hope it is only a misleading help text. A complete description will be more complicated. An unconnected index input is not always a constant. If one index input is connected to a variable, all the following unconnected index inputs will also be variable.
True, but I think what matters is if the compiler can determine at compile time which index is used (although the help does not explicitely say so). If the compiler finds the index is constant it will be a free operation on the FPGA.