The position is identified by the waveform of electromotive force (induced voltage) generated from the rotor rotation.
Only a square-wave drive can detect the rotor position by the induced voltage.
One set of coils among U phase, V phase, and W phase always has the current turned off. By detecting the point at which the amplitude of the motor drive waveform becomes 1/2 in its non-energized phase, the position of the rotor can be specified every 60 degrees.
The drawback of this approach occurs in the stop state. No position can be detected because no electromotive force is generated.