“Never used an Unforgivable Curse before, have you, boy?” she
yelled. She had abandoned her baby voice now. “You need to mean
them, Potter! You need to really want to cause pain — to enjoy it —
righteous anger won’t hurt me for long — I’ll show you how it is
done, shall I? I’ll give you a lesson —”

“They’re Imperiused,” he added, in
response to Hermione and Ron’s confused queries about Travers and
Bogrod, who were both now standing there looking blank. “I don’t think I did it strongly enough, I don’t know . . . ”
And another memory darted through his mind, of the real Bellatrix
Lestrange shrieking at him when he had first tried to use an
Unforgivable Curse: “You need to mean them, Potter!”