P. 773 – q. 18 (70)

Though best known as a jazz vocalist, he also enjoyed gospel music, whereby he told his manager that he wanted to make a recording of his favorite gospel songs. No error

‘The name “transferware” comes from a 19th century technique [according to which] a pattern is engraved onto a copper roll, printed on tissue paper,and transferred onto earthenware dishes’

[according to which] is wrong, and should be replaced by [whereby]. Can you precisely please explain why? Thanks!