Sometimes it happens when the teams are "on the clock" but it usually happens about 5-10 picks after the original team picked. Sometimes even later. I beleive the Pacers announced the trade for G.Hill/Kawhi during either the late 1st rd or early 2nd (if I remember correctly)even though he was selected #15....
“I think he’s going to be a star. And as time goes on, he’ll be the face of the Spurs, I think.” ---- Coach Pop on Leonard