Two dials set the qubits as if they were independent (a product state). The entanglement dial mixes in correlation. Watch the joint amplitudes stop factorizing.
The solid arrow on each circle is u0 (the direction you drag); the dashed arrow is u1 (90° rotated). Both arrows' lengths are Schmidt coefficients: solid = cos η (weight of "both at u0"), dashed = sin η (weight of "both at u1"). At η = 0 the solid is full and the dashed is gone: the state is purely "both at u0," a product. Turn up η and weight flows from solid to dashed: the state becomes a mix of "both at u0" and "both at u1." At η = 45° both arrows are equally long (0.71 each): maximally entangled. The formula above shows the same numbers; the arrows are its visual.