His heart takes off like a trapped bird

[shootingstar]

(William, the chorister, is getting the stick)

Now he's actually standing there, William wishes he'd asked Martin (his friend), who is waiting his turn outside the study, to tell him exactly what was going to happen.
Mr. Atkinson stands next to William, and his heart takes off like a trapped bird. What now? Does he bend down? Does he wait to be told to bend down? How far does he bend? All the way? Halfway? Why the hell didn't he ask Martin?
'Bend over.' . . .

(From A Terrible Kindness by Jo Browning Wroe, Part II Cambridge Choir, chapter 30)

Please, what does 'take off' mean there?

 

[teechar]

It means leaps, or kicks into action. It's metaphoric.
In other words, his heart starts beating faster.