The light curves of low-mass X-ray binaries show variability on timescales from milliseconds to months. The shorter (sub-second) variability is particularly interesting because it is thought to probe the inner region of the accretion disk and the central compact object. X-ray spectral-timing is a new type of analysis that seeks to investigate how matter behaves in the strong gravitational field around the compact object. Observations suggest that different types of low-frequency quasi-periodic oscillations (QPOs) are associated with different emitting-region geometries (e.g., disk-like or jet-like) in the innermost part of the X-ray binary, that are varying possibly due to general relativistic precession. We developed a technique for phase-resolved spectroscopy of QPOs, and are applying it to two types of low-frequency QPOs from the black hole X-ray binary GX 339-4. Over a QPO “period”, we find that the energy spectrum changes not only in normalization, but also in spectral shape. We can quantify how the spectral shape changes as a function of QPO phase, and the two different QPOs show markedly different spectral changes. This talk will feature results from this paper, as well as new preliminary results.