This demo set shows two methods for obtaining the Helix Cannon on level two. There are two switches that must be tripped, and the second one (the one that opens the external access to the secret room with the Helix) is a one-shot. This means that here are no second chances. If you don't get through the door in time, you won't be getting the weapon. That is, unless you restore from your saved game, of course! :-)
In both methods, the first switch you must trip is visible through the grates from the "hallway." This opens an internal access panel to the Helix room.
The first method is the most straightforward and probably the one the level designer had in mind. In this demo, I take extra time to instructively point out the key features as I go.
First, I trip the visible switch from the hallway. Then I swing around to the hallway on the other side, stopping to show the location of a secret door that I cannot yet open. Continuing down the hallway, I then expose a secret room that is accessed through the ceiling. This leads to the secret compartment opposite the Helix room. In it, you'll find a guided missile, energy boost, and cloaking device. I pick these up for good measure, but it's not required to do this to get the Helix.
Also in this room, near the (secret) door, is another switch. Tripping this switch will open the secret door I pointed out earlier. Once opened, you have a short time to get through this door before it closes. Once it closes, it's closed for good.
So, I exit the "cloak" secret compartment and take aim at the switch from below, in the "public" tunnel, poised to make a B-line for the other secret door. I trip the switch, and I'm off. I get through the door just as it's closing. Afterburner is not required but may provide a safety margin. Be careful though, as it's a little harder to navigate under the afterburner, and the hallway is twisty.
This demo shows a rather satisfying but wasteful way to accomplish the task. In it, I presume you've watched L2Helix1, and I don't take any time to be overly instructive. I just execute the required maneuvers.
I say it's wasteful because you burn (at least) one guided missile. I can tell you that they will come in handy in the later levels!
At any rate, as required, I blow the visible Helix switch first. Then I guide a missile through the opposite grate, into the "cloak" secret compartment, down the tunnel, and into the concealed switch. As you now know, this opens the external door to the Helix. I then high-tail it to the other hallway and up through the secret door.