On first applies an external magnetic field, this splits energy levels due to the Zeeman effect. Hyperfine splitting splits these further. Looking at the absorption spectra the peaks are equivalent with microwave photon energies perfectly resonant to bridge the energy gaps generated by hyperfine and zeeman splitting. With this energy value one can calculate, using the value of the magnetic field and some quantum numbers, the g-factor. Doing this over multiple angles of the applied magnetic field, or rather by rotating the sample, instead of just a g-factor, the whole g-tensor can be found.