cursed necklace

[siddhu1616]

No: HPFGUIDX 148566

Hey all,
I've one question:
Madam Rosmerta was under imperius curse for nearly whole year;
in the chapter "Silver and Opals," Leanne (Katie's friend) said 
that Katie was under imperius curse when they were arguing about 
that cursed necklace!

But my question is how can a person under imperius curse uses a 
curse as the the one who gave the imperius curse was not present 
to see there on whom the cuse was used!

I know in Goblet of Fire Krum had used curses on Cedric and Fleur 
but the one who gave him curse (Mad-Eye) could see from bushes on 
whom he was using it! As he had seen Harry and Cedric before first 
task! Draco was surely not there so how Katie had been attacked?
Do you have answer?

Please reply, 
thanx.

Sid