The white queen must keep guarding against the check on b7, so 1.Qb3 threatens 2.Qg8. After 1...e6 2.Sxe6 threatens 3.Sxc7. 2...dxe6 is met by the switchback 3.Qb5 threatening mate at e8, while 2...Rc1 fails to 3.Sxc7+ Rxc7 4.Qg8+, so the best defence is 2...Bh2. 3.Sf4 renews the threat at g8 and after 3...Rg1 the point of the problem is revealed – 4.Sg2, threatening mates at b7 and g8, a Nowotny interference at maximum distance from the black king. The construction is very clear.