Complex sentence: Birch has investigated the evolution of the meaning of the Sanskrit word haṭha, and in particular the key role of the "Haṭhayogapradīpikā" in popularizing a particular interpretation of this term.
Split into simpler sentences: Birch has investigated the evolution of the meaning of the meaning of the Sanskrit word Hatha. He specifically researched the key role of the Hathapradīpikā in popularizing a particular interpretation of this term.