When the needle bar and needle are at the highest point, the hook of take-up is back.
As the needle and needle bar descend, the hook of take-up swings forward . . .
. . . And by the time the needle has reached it's lowest point, it has swung back again, leaving quite a bit of slack on the spool thread.
As the needle bar rises, the hook of take-up swings forward again, taking up the slack on the thread.
And by the time the needle has reached it's highest point, the hook of take-up has returned to the back again.