A brief introduction is on the Photon Science page:
https://photon-science.desy.de/facilities/petra_iii/beamlines/p64_advanced_xafs/infrastructure_and_experiment/qexafs/index_eng.html
and in the following paper:
https://aip.scitation.org/doi/10.1063/1.5084609
A lot of implementation details can be found in the PhD thesis of Oliver Mueller: