We show that the Second Derivative (SD) technique of the absorption spectra of Rb atomic vapours confined in a nanocell with a thickness = λ/2 = 398 nm allows to achieve close to Doppler-free spectroscopy. Narrow linewidth and linearity of the SD signal response with respect to transition probabilities allow us to study separately, in an external transverse magnetic field (0.6 to 4 kG), a big number of the atomic transitions of 85 Rb and 87 Rb atoms. Atomic transitions |F g , 0 → |F e = F g , 0 , for which the dipole moment is null at zero magnetic field (so-called magneticallyinduced transitions), show a gigantic increase in probability with increasing magnetic field. When a magnetic field is applied on the vapour, we show the possibility of forming a dark resonance on these transitions by adding a coupling laser. We hence demonstrate a five-fold increase in the transmission of the probe radiation when the coupling laser is on. Theoretical calculations are in a very good agreement with experimental results.