Lookdown is a technique used in speedruns of GoldenEye 007 (and, to a much lesser extent, Perfect Dark). The principle behind the technique is that if you are looking at the ground, the game has fewer polygons to render, so you move faster. This doesn't quite make sense as an explanation, but it is a fact that lookdown saves approximately 1 second for every 75 seconds of play. It has hence become a critical factor in breaking several world records which were previously thought to be maxed.
Note that it is not usual to look straight down, and there is probably not much more gain in doing so. What is more common, however, is adjusting the view approximately 45 +/- degrees down towards the ground so that it is easier to see the course.
The fact that looking down makes it harder to see where you're going means lookdown is most useful on flat speed levels with few enemies and sparse objectives, like Surface 1, Surface 2 and Streets. That being said, lookdown is commonly used on most GoldenEye 007 levels, but of course not continuously throughout full runs. Interrupting lookdown by switching between looking at the standard plane and looking down is often done numerous times during runs.
Another effect of lookdown, besides the time saved, is that turns can be done slightly faster and sharper. Taking advantage of this side-effect especially applies to sharp corners and places where multiple obvious turns occur.
Lookdown was first brought to the attention of Goldeneye players by a user called Supernova, who claimed at the time to be a 55 year-old man named John Kaleta. Supernova would also reveal the technique of creating a makeshift crosshair with tape or marker to the Goldeneye community, since the game did not feature an in-game crosshair for aiming, which he called the "Illinois Cross of Death."