it makes you stay over the bars so the scooter is way away from you and you don't loop out. Almost every trick you should have more weight over the front rather than the back. The farther forward the headtube angle I've it's easier to be over the bars.
Take this for example, you can whip right?
Go outside and tailwhip with your head straight with your bars. Easy peasy right?
Now try to stick your butt out like you do for manuals and do a tailwhip. Not so easy now?
ummm, it doesn;t matter if you lean forward, just keep the deck under your feet. Then just repetition till you can catch 'em, double whips and so forth