The vibration directions of the light in the acute bisectrix interference figure can be derived in a similar manner to that used for uniaxial minerals. By taking a series of slices through the indicatrix, at right angles to the wave normals, we can determine the vibration directions for all paths of light emerging from the indicatrix.
The image below shows a biaxial negative indicatrix, X is the acute bisectrix, on which the vibration directions for the light are plotted as the blue lines on the surface of the indicatrix.
Since we are looking at the interference figure for the acute bisectrix, the indicatrix must be rotated so that the Bxa is vertical. This rotation causes the vibration directions to rotate as well. In the image below the indicatrix is shown, in grren, inside the mineral, and the vibration directions for the light are indicated, in blue, on the surface of the indicatrix.
The vibration directions are projected onto the surface of the mineral and then onto the interference figure.
In the interference figure where the vibration directions through the mineral are parallel to the polarization directions of the microscope the resulting interference colour in the interference figure will be black - this is the isogyre. (I have not shown the isogyre in this image, so that the vibration directions of the light can be seen.)
On rotating the stage, the position of the vibration directions on the indicatrix surface will be rotated resulting in the change in the shape and movement of the isogyres in the interference figure.