The input is very quick. The idea is to, after pressing b to do any of your specials with any character you very quickly tap in the other direction. You will turn around and preserve your old momentum in the new direction while doing the move in the other direction. look up videos about 'wavebouncing' that show the player's hands. SpiderMad has some up earlier in the thread I believe. (at least I think that it's him, he seems to post videos from that channel a lot, I'm not that familiar with him heh.) to try it out for yourself, go into practice mode, put the speed to 2/3 or 1/2 or whatever you find that works, and try the method listed above. I personally find it the easiest to do with a forward b consistently, but you can do it with any b move that doesn't completely negate your momentum. So Lucas's up-b is out (except perhaps if you run off the ledge with it, then you might be able to, because it preserves his momemntum when timed right), but his neutral, forward and (pre 2.6) down-b could all do it. you could do some really cool momentum tricks with it. here's a crappy quality playlist of me using that plus the trick with using PK Freeze to go underneath every stage that's go-underneathable. (it's most certainly not applicable in matches except as one hell of a risky mindgame/taunt lol)
http://www.youtube.com/watch?v=nYyfgzMSGqQ&list=PLjXLROPm38cVgdPD6j7Z2Bw88nZ9bzFoP
What I'm doing is running off the stage, doing a b-reversal (wavebounce, whatever) forward-b to turn around and start moving underneath the stage, then a specific amount of time after using the forward-b, I used down-b, and that gives me a huge momentum boost and allows me to reach the other ledge. You used to be able to wavebounce the down-b, after the forward-b, to suddenly turn around and shoot in the opposite direction you were travelling. You could use it to edgeguard then quickly grab the ledge without even using your double jump.
Btw the reason I keep saying "DONE!" is that I got really frustrated trying to get the Final Destination vids, and decided to do all the others first because they were simple by comparison. I ended up giving up on getting the FD in the same format as the others and did it in two separate vids lol.