Low frequency quasi-periodic oscillations (QPO) with periods between ~10 s and ~0.05 s are often seen in the X-ray flux of accreting stellar-mass black holes. These are often attributed to the Lense-Thirring precession of the inner accretion flow, a General Relativistic effect caused by the spin of the black hole. QPO phase-resolved spectroscopy (measuring how the X-ray spectrum changes with QPO phase) provides the best test of this model against alternative interpretations, since precession leads to a rocking of the ~6.4 keV iron fluorescence line between Doppler red and blue shift over the course of each QPO cycle. I will present the latest results of our QPO phase-resolved spectral analyses, including the bright black hole X-ray binary system GRS1915+105 for which we utilise simultaneous data from NICER and NuSTAR. The high count rate and broad band X-ray coverage (~0.3-79 keV) provided by the combination of NICER and NuSTAR made this an ideal dataset for our analysis. I will discuss the statistical significance of the asymmetrical illumination profile and of the ‘rocking’ iron line, and the interpretation of these results.