Henry Watson Fowler, The King's English (1906)
“ In both constructions, the 'that' clause, supplemented or introduced by 'it', gives us the subject of a predication, the relative clause (with it) being equivalent to a pure noun, the conjunction clause to a verbal noun in apposition, partly retaining its verbal character. In both, also, the predication answers an imaginary question, recorded distinctly in the relative, less distinctly in the conjunction clause. ”
