Hello is used to show a two-suited hand with spades. Thus, after opponents' bid one notrump, partner may interfere by bidding

2C forces 2D...u can then pass with Diamonds, but if u then bid a major
you have that major and either minor
2D a transfer to hearts (showing hearts)
2H is the majors
2S is spades
2NT is a transfer to clubs (showing clubs)
3C is for both minors
3D is for both majors but very strong
Double is strictly penalty versus strong NT, but should just show cards vs weak NT

Source: Dave Smith