This comes into its own when playing sounds based on fractal orbits, producing results from haunting chordal pulses through to unearthly arpeggios. Fractint then waits for one orbitdelay period, calculates the next orbit, assigns a frequency to the second voice, and triggers it on. This continues until the number of voices sounding is equal to the polyphony value whereupon the first selected voice is triggered off and the note allowed to decay. That probably all sounds horrendously complex but just think of it as playing the organ, holding down each new note for as long as possible but with the polyphony value controlling how many fingers you have. This controls the time taken for a note to hit full volume after being triggered on.
Low values give a punchy percussive sound while long values give a softer bowed sound.
If attack time is set too long and orbitdelay is low then notes are played too fast to ever achieve any volume and the net result is low or non existent output. To put it another way, if orbitdelay is low then this value should be low also. This controls the time taken for a note to die down to the sustain level. Setting this time too low may produce a rhythmic click between notes. The attack and decay values don't allow much fine control.