Fixed Wing Mode is a relatively new flight mode that was introduced to the Mavic Pro in April 2017.
Before we go any further, let me just say this: Fixed Wing Mode is quite different to any other mode on the Mavic.
It is kind of like flying your drone like a plane. Instead of left and right just moving to the left/right, it "banks" like a plane does when it turns. So, at an angle (to the horizontal).
The same for up and down. They are more like climbing and diving in a plane, than just moving straight up or down as you do with a quadcopter like the Mavic.
The Mavic does all of this by keeping the gimbal fixed, which helps to give you that "flying a plane" feeling that you will get.
Here is a video to show you what some turns and dive/climb look like in Fixed Wing Mode:
Unlike most of the other flight modes on the Mavic Pro, you have to enter the controller settings menu (via the Drone Icon) to get into Fixed Wing Mode.
You can see how it looks in the settings menu below:
Ok, so now you are in Fixed Wing Mode, but we are not there yet.
In order to "use" fixed wing mode the Mavic has to be flying at least 3m/s (about 10ft/s). This is for safety's sake.
So, first make sure you are moving at a reasonable speed then you will be able to use Fixed Wing Mode.
Then you have to click on the "C1" button on the bottom of your controller. This is underneath on the left hand side (the image below shows it on the right because you are looking at the controller from the front).
As far as controls go, things are now a little different. We are focused mainly on the left hand side of the controller here:
Note: For any of your pilots out there, this is quite different to flying a plane where the up and down would be reversed on your normal plane joystick.
Right Hand Side:
I think DJI brought the Fixed Wing Mode out to be compatible with the DJI Goggles.
When you are in Head Tracking Flight Mode using the DJI Goggles, the Mavic responds like it does in Fixed Wing Mode. Except, in this case, you are using your head as the steering mechanism. This way, it is a lot more immersive and a cooler experience. More like flying and being inside a plane, but just using your Mavic!