The lip and it's shape - is pretty much a matter of what looks good and is easy to do -
Iwould guess some eons ago - air tuft tests showed that the accepted grouping and shape of louvers - be it on the hood of a 32 chopped coupe or the YAK- is "close enough"
The lip causes a turbulance at the aft edge - that's all that is needed - it makes a low pressure and Mother Nature tries to rebalance it -causing air to flow faster to that spot.