creates a list with two named entries for the parts of the signal Speed ? 0 and ? 0. The parts have the same size as the original signal, the values not satisfying the condition are set to the floating point void value (? or NaN), as specified by the third argument EVENT_SETVOID. Other operations can be looked up in the Online Help (Index: ValuesAboveLevel (FPScript)).
One can achieve the same results by using the Event Isolation on the desired signal avoiding the need to write any FPScript code.