Stealth_Force got it right but I need to add that trying to shoot straight down normally puts you in an incorrect position to shoot through your peep properly. Not to mention the fact that spine shots or one lung shots with that angle are low percentage shots.

Personally I've passed in straight down shots in hopes of something better. This year I had an 8 pointer come straight at me and I watched him walk straight under my stand. However "The Can" did him in and he came back and gave me an 18 yard broadside.

Come on guys, if the deer is not somewhat broadside, let it walk. With luck it might give you something better a few yards out.