More detail straight from the horse's mouth:

"Do you plan in advance for songs to fade out?"

Geddy Lee (Guitar Player, April 1986): "Sometimes. Sometimes we're not sure, so we ride out for a long time, and then end it. We have the option. Invariably, every time we decide we're going to fade out, we start getting into the fade and everyone loosens up and the track starts getting better. That happened with "Mystic Rhythms" [Power Windows]; the fade-out is about a minute long because we liked every little nuance. The end of "Grand Designs" [Power Windows] is also like that. There are about seven phrases, and they're all different. None of that was planned; Neil was doing the drum track, and at the end, the sequencers were going and he just kept punching-in and going, basically flailing and hacking through it. Everybody loved it, so we decided to keep it in. Then we had to learn to play it onstage."