Black hole astrophysics can be regarded as a fundamental tool for us to learn about accretion and ejection physics in the strongest gravity regime in the Universe. With only a few black holes that can be resolved spatically, time domain techniques are very powerful to study the inner region of black holes. In particular, reverberation mapping measures light echoes off the inner accretion disc near the innermost stable circular orbit. I will present some of the highest quality reverberation mapping results of a stellar mass black hole to date, from NICER observations of the 2018 outburst of MAXI J1820+070. For the first time, we measure reverberation lags during the hard-to-soft state transition, and find that during this time, the frequency of the reverberation lag decreases, suggesting a larger emitting region, with a possibility of an expanding corona. We jointly fit the lag-energy spectra with the reverberation model RELTRANS for a range of Fourier frequencies in each epoch, and find an increase in the coronal height, consistent with the qualitative expectation. I will put these results into context, comparing to reverberation in other low mass X-ray binaries, and discuss implications for unanswered questions in black hole astrophysics, including the state transition mechanism, system geometry and coupling between the disk, corona and jet.