Halo Runs record page: http://www.haloruns.com/records?lb=109 (click the timestamps for video links)
A tutorial for this level on legendary can be found here: http://www.twitch.tv/scurty_/c/5275414
To perform the shield bump, zap the first shield generator in the level, then position yourself on top of it in exactly the right spot. When the shield reforms, you will be pushed above the level, out of bounds. Then you can walk backwards to the bridge, drop in, hit a load trigger to load the enemies, walk back and start the bridge cutscene.
The positioning is different between Xbox and PC, and on PC, it differs depending on your aspect ratio. Jumping and airstrafing allows for finer positioning than walking. Shooting the shield generator gives you more time to position yourself. There is about 3-4 pixels of leeway, making this one of the most precise tricks in the series.
After the shield bump, make sure to grenade the enemy groups to avoid dying in the cutscene / on your way out.