When to start the clock

[NothernVA_Ump]

I had a fellow offical tell a clock operator that on a missed free throw (1 and 1 or last free throw) that the clock is to start when it is touched. I have told the clock operators that the trail will signal (with the chop) when they are to start the clock on a missed free throw. When they ask when is that, I say that it is when the free throw is clearly missed (is under the rim); but not to determine it for themselves, just to watch the trails arm. Who is correct?