Complex sentence: It featured a fast, intense drumbeat which Discharge emulated with distinctive D-beat starting 1979 before changing their line-up and beginning a new style indebted to heavy metal.
Split into simpler sentences: It featured a fast, intense drumbeat which Discharge would later emulate in the distinctive drumbeat of D-beat. In 1979, the group changed its line-up and began a new style indebted to heavy metal.