1) Go in the editor and place down a vehicle with a turn-out FFV position, such as the M2A1 Slammer
2) Ensure that the vehicle is set to "player as commander" so that you have access to an FFV position, and preview the mission
3) Once you've spawned in the game in the commander position of the tank, proceed to turn out from the turret using the action menu or keybind
4) Notice that if you try to raise your weapon to fire by pressing the "raise/lower weapon" keybind or pressing the "fire" button" like you would in an FFV-enabled seat, or try to throw a grenade, nothing happens. Also pay attention to the animation that the player is in.
5) Turn in and out a few times to see that FFV doesn't simply enable after turning back in.
6) While turned out, press your "handgun" key and the player will unholster their handgun slot weapon. You can now use the FFV capability of the position properly, changing back and forth between primary and secondary weapon, throwing grenades etc. etc. Notice that if you lower your primary weapon, the animation is different to what it was before.
7) Start turning in and out again and you will notice that you begin in the "weapon lowered" stance and can immediately FFV every time you turn out from now on.
8) Go back to the mission editor and this time set yourself as a 'Man' class unit and place an empty Slammer down with a vehicle name (e.g. call the Slammer "T1"
9) In the init field of your 'Man' class unit, type "this moveincommander T1;" (without quotation marks) or replace "T1" with whatever you named the Slammer. Then preview the mission.
10) Once spawned inside the tank's commander position, turn out again and notice that you start in the "weapon lowered" animation for the FFV position rather than the static pose you had first time. You will notice that you can immediately raise and lower or fire your weapon, or throw grenades without having to draw your handgun first to enable the FFV capability.
For a final couple of things to try:
Preview a mission where you get in to the Slammer from the outside at the start of the mission rather than spawning inside it. You will be able to use the FFV capability as soon as you turn out, without drawing a handgun first.
Go back to step 1, but this time spawn on the AAF side as the commander of a Kuma. When you turn out you'll see that your gunner is in the FFV "weapon lowered" state while you're in the static animation unable to FFV. Switching between commander and gunner will still not allow you to FFV until you draw your handgun or get out and back in to the vehicle.