Wind up your shoulders and kick the tail as hard as possible. Ultimately, it's not wrong. But that alone can't explain this trick.
Summary
Kick the tail forward, toward the front of your body
Kick the tail toward the front of your body because that's how the human body generates the most power. When it's facing sideways, it can't produce enough force.
Keep looking at your back foot as you pop
You can easily overrotate upon kicking the tail. But you can force the direction you kick by looking at the object you want to kick.
Have your weight near the rear truck
By having your weight near the rear truck, the tail comes in front of you as you pop, allowing you to kick it forward. But if you have it further forward, you can't keep the tail in your sight, and the kicking power weakens.
Why does the board spin?
The simplest answer, obviously, is that it receives enough energy. But even if you kick the board as hard as possible, it often doesn't spin. And it's not because my kicking power is stronger than yours. The secret behind this problem lies in the structure of the human body. Naturally, kicking power is maximized when an object you want to kick is in front of you. In football, for example, when you want to kick a ball strongly, you'd place it in front of you.
Take a look at my movement. When I pop the tail, although my body and board have already started rotating, the tail stays in front of my body. As a result, the board hits the ground strongly and pops up into the air, while receiving enough rotational force. It is quite obvious that the kicking power is stronger when the object is in front of you, but in Frontside tricks, everyone suddenly forgets this fact.
As you have to complete the rotation, you tend to focus too much on rotating your upper body before spinning the board. Then, your back foot ends up kicking the tail outside your view. This is the same as trying to kick a soccer ball while facing backwards. Exactly how much power can you possibly generate like that? Based on the structure of the human body, it is obviously ineffective.
Meaning, it is important to keep the tail within the "Sweet Angle" in front of your body, where you can maximize the strength of your kick.
Paradox of shoulder rotation
Now here comes the paradox. While popping in the Sweet Angle maximizes the kicking power, you also need to turn your shoulders to complete the rotation of your body. So, you can't overrotate your shoulders, but you do have to rotate them. The key to solving this contradiction is in the stance while crouching. Your upper body should face completely sideways or even further back.
As you extend your body, turn your shoulders about 45 degrees while kicking your back foot forward. Between the moment you start pushing down the tail and it reaches the ground, the board rotates on the ground with your shoulder, allowing you to kick it right in front of your body. As the board and body start spinning before the tail hits the ground, you can solve the paradox by being able to open your shoulders to rotate your body, and keep the tail in the Sweet Angle to maximize the kicking power.
The line of sight
Still, it's really hard not to overrotate your body, knowing you have to do a 180. The line of sight helps you force the direction you kick. Until the moment you pop, keep your gaze on your back foot. By keeping an object you want to kick in your sight, you can literally focus your energy on it.
However, you won't be able to see the direction you are heading by looking at your back foot, unlike in an Ollie, where you see your front foot while still catching sight of what lies ahead. Getting used to this feeling requires some practice.
The center of gravity
In an Ollie, you keep your center of gravity near the center of the board. However, in a Frontside Bigspin, you shift it closer to the rear truck. By doing so, the reaction force from the ground passes directly through the rear truck into your center of gravity, allowing you to gain more airtime.
And it's not just about the airtime. The position of the center of gravity also greatly affects how fast the board spins. Combined with the rotation of the body, kicking the back foot forward pushes your body weight backward. Then, the tail naturally swings out in front of you. Notice the tail is behind you at first, but it moves forward relative to the center of gravity by the time it hits the ground.
If the body weight is placed further forward, the tail stays behind you. Your body has already turned and is facing diagonally forward. So the direction you kick will be outside of the Sweet Angle.
The position of the weight might be a lot farther behind than you might think. I personally tilt my front knee inward. Compared to simply narrowing the stance, this pushes my weight over the rear truck. Try different adjustments to find what works best for your body.
Stance and foot placement
They often say you should place your front foot in a Heelflip position, with the toe hanging out, and put your back foot on the toe side, similar to a Frontside Pop Shove-it. As the back foot applies pressure on the heel side, placing the front foot on the toe side is scientifically reasonable, as it works as a counterweight to keep the board level.
The angle of the front foot is not so important, however. Personally, I put my front foot slightly open. This is because my ankles are stiff, and if I place it at a right angle to my board, I cannot crouch comfortably. By opening the angle slightly, more of my front foot remains in contact with the board while crouching, making it more stable.
For the back foot, you may place it just as you would for a Frontside Pop Shove-it. Since the role of the back foot is to kick the board, and you are not twisting it inward, there is no need to angle it excessively.