Complex sentence: Basically, it was a chordophone not unlike a clavichord, that is the strings were struck, not plucked; but the suspension and the tautening of the numerous metal strings (it is said there were 790 ones!) were much more elaborate.
Split into simpler sentences: Basically, it was a chordophone not unlike a clavichord--in other words, the strings were struck, not plucked. However, but the suspension and the tautening of the numerous metal strings (which, it is said, numbered the astonishing total of 790) were much more elaborate.