Fakie Bigspin. The mechanism itself should be simple enough: A combination of a Shove-it and a body rotation. But it's surprisingly easy to stop halfway when you know you are popping as hard as possible. Don't worry, there is a scientific way to solve this problem.
Summary
The front foot guides the spin
You can't spin all the way only by popping the tail with your back foot as the front foot interferes with the spin. Open your body while guiding the board with your front foot until about 120 degrees as if the front foot sends the board forward.
Move your body weight from the center to the front
Place your center of gravity in the middle of the board while crouching. As you extend your body, move it toward the direction you are heading. Doing this allows you to let the board slide underneath your body weight as you scoop.
The Front Foot Also Spins the Board
Unlike a regular Shove-it, it's not only your back foot that spins the board in a Fakie Bigspin. A common mistake, especially in tricks like Fakie Bigspins or Nollie Shove-its where you spin the board by popping the front part of it, is trying to spin the board only with the popping foot. When you push down the board with your popping foot, the rear side of the board naturally tries to lift — but your back foot interferes, canceling out that rotational force. That means the pop alone isn't enough to spin the board.
Instead, your not-popping foot needs to move with the board while guiding its rotation partway through the motion. In a Nollie Shove-it, for example, the back foot guides and follows the board through the initial part of the spin. Likewise, in a Fakie Bigspin, if you watch in slow motion, you'll notice the front foot rotates together with the board until about 120 degrees of rotation. In a regular Shove-it, only the popping foot gives rotation to the board, and the front foot practically does nothing to spin it. Understanding this difference is the first step to making your Fakie Bigspins smoother.
The back foot gives the board further spinning energy
Of course, most of the rotational force comes from swinging your back foot backward. Your hip, knee, and ankle must work together.
The shoulders spin the body
Before popping, wind up your shoulders, then unwind them as you extend your body. This motion creates rotation throughout your body. As your body spins, your front foot continues to guide the board, keeping contact until the board reaches approximately 120 degrees. If your Fakie Bigspin fails, check whether your front foot is drifting too far from the board at that moment.
The weight distribution and the body axis
When crouching, keep your center of gravity above the middle of the deck. As you extend your body, slightly shift your weight forward. This is because when your back foot scoops, the board naturally moves forward. By moving your weight forward in advance, you will be able to let the board slide underneath you.
The flipping problem
The board tends to flip unintentionally. If you pop too high, you'll give the board room to flip. So, keeping your jump low will make the trick much more controllable.
An alternative way
If you are having trouble, trying a different version might give you something new. Open your shoulders and revert first, do a light manual, and then scoop the back foot. This might not be a traditional Fakie Bigspin, but hopefully gives you something useful.