NaN green lines have the same velocity as 1x inherited points, but they have the neat effect of removing slider ticks. This allows Mapping Tools to multiply slider velocity by insane values using BPM, and it does this without having to deal with equally insane amounts of slider ticks.
If you want a slider to go 500x faster (which, in the case of aspire mapping, can be on the low end), you're obviously limited by green lines being restricted to 0.1x-10x. Increasing the BPM also allows faster sliders, but even with a maxed out 10x green line, you still need to multiply your BPM by 50. Fifty times as many slider ticks. Yuck.
So, the best option is to multiply the bpm by 500, then use a xNaN greenline to eradicate those slider ticks in one fell swoop. A 500x faster slider with no slider ticks! Magical. In practice, along with changing the BPM for single milliseconds at a time, this leads to surprisingly few side effects. Slider 'fects, if you will.
(NOTABLE SIDE EFFECTS : BUZZ SLIDERS BECOME IMPOSSIBLE TO HIT. FIRST OBJECT OF A MAP BEING ASSIGNED AN NaN VALUE MAKES MAP IMPOSSIBLE TO LOAD OUTSIDE OF THE EDITOR. SEVERE RADIATION POISONING IS ASSOCIATED WITH REPEATED USE. EDITOR WILL CRASH WHEN SELECTING AN NaN GREENLINE IN THE TIMING WINDOW. USE WITH CAUTION.)