Complex sentence: Residbegovic developed many instrumental techniques, but the most important and innovative in music of the 21st century are definitely his piano techniques and treatment of piano strings and resonator of the instrument such as use of synthesizer in contemporary classical music.
Split into simpler sentences: Residbegovic developed many instrumental techniques, but the most important and innovative in music of 21. Century are definitely his piano techniques and treatment of piano strings and resonator of the instrument such as use of synthesizer in contemporary classical music.