Grano Astro is a convention for showing your suit(s) after your opponent has opened 1N.
Here are the bids and their meanings:
This convention differs from others such as DONT and Hamilton in that those show a minor and a higher suit while Grano Asto shows the more important major suit and a lower suit.
Another difference is that at least one suit is specified for each bid, while other conventions have bids with suits to be named later, such as Dbl or 2C showing any 1-suited hand.
If NT's partner jumps up the bidding, Grano's partner is likely shut out of the bidding because he cannot know if you have a fit or not.
On the other hand, leaving suit(s) unnamed makes it harder for NT's partner to bid precisely. For example, he cannot double for penalty or show a stopper in your suit if he doesn't know what your suit is.
On the surface, not naming a suit seems like it would hurt NT's partner more often than interferor's partner.