Take a look at the original "Return to Sender" cutscene. Done? Cool.

So, the Covenant ship fires on the UNSC ship, right? Its shields are down. Two Longswords fly in and blast a hole in the thing while its shields are down. This potentially keeps those shields from coming back up, at least in that area. Master Chief rides inside with the bomb, arms it, bounces out. Boom.

This trailer essentially has the shields up. Bad news. Odds are you just killed the Chief, or allowed that ship to survive.

Or maybe the actual cinematic will show that, and this little mishap was because I'm only supposed to care about your new faceless drone and Arbiter. Whatever. Let's try and keep it consistent and like we actually know about the universe and care about it beyond money.