How on earth do you practice it? You can practice bench pressing by pressing a small weight. You can practice boxing by sparring.
You can't just climb up to a deadly height - which isn't even very much, as people regularly die from falling out windows - and work your way up from there.
You can't just do a little batsuit jump, either. You have to be going fast for the suit to make a difference.
Most free solo climbers climb with a rope for the majority of their climbs, although exceptions like Paul Preuss exist. They often rehearse free solo projects many times on rope before going free solo, although there a notable cases of 'onsight' free solos like Hansjörg Auer's 'Weg durch den Fisch'. In the case discussed here, Alex Honnold apparently climbed Freerider around a dozen times on rope and rehearsed the crux pitch many more times. He did take several falls on the route, one of them spraining his ankle last summer.
http://www.planetmountain.com/it/notizie/arrampicata/hansjor...