When the ball is touched inbounds.
NCAA rule 2-11-5
Duties of Shot-Clock Operator
The shot-clock operator shall:
Art. 5.
Start the timing device when a player inbounds legally touches or is
touched by the ball on a throw-in or when a team initially gains possession
from a jump ball, an unsuccessful try for goal or when possession is gained
of a loose ball after a jump ball or unsuccessful try for goal.