to add front downforce? i think i know the thing you're referring to... and if i'm thinking of the right thing - when you think about it, if it's angled down at high speed, it's going to push the front of the car down...
like ---> /
but if it's angled up at low speed...
like ----> __ (but say, raised up at more of an angle, duh )
then it's not going to make as much downforce, as it's not needed at those speeds...