a 360 downside whip is when you tail whip one way, and then 3 the other, meeting the bike half way and then catching it, rotate the with the 3 to finish it off. A decade is were you kick off the bike and twist around the front a complete rotation instead of meeting the bike at 180 degrees, or backwards. basically what Ian said.
That's a very good question that I'll be going a little deeper into in the next little while. I talked to a few of the riders about it and the opinion seems to be that in a decade the bike stays straight and the rider goes around the bike and in a downside 360 whip the rider and bike start to spin and the rider kicks the bike around while he spins the other way and meets the bike part way around. So basically it seems to be a matter of bike movement...