It shows that reference frames are independent physical spaces.

What Is the 3D Space-Time Diagram

In 3DSD, the real δ = 0°
Minkowski's imaginary ψ = 0°

The 3D Space-Time Diagram In Action 0

In 3DSD, the real δ = 30°
Minkowski's imaginary ψ = 4.1°

The 3D Space-Time Diagram In Action 30

In 3DSD, the real δ = 60°
Minkowski's imaginary ψ = 18.4°

The 3D Space-Time Diagram In Action 60

In 3DSD, the real δ = 90°
Minkowski's imaginary ψ = 45°

The 3D Space-Time Diagram In Action 90

The 3D Space-Time Diagram In Action 0

In 3DSD, the real δ = 0°

Minkowski's imaginary ψ = 0°