When does the imam say the takbir for the sitting of rest?

If he says the takbir before the sitting, the worshippers will precede him. There is nothing wrong with that; the imam, it means, the sitting of rest is just for a few moments.

If the worshippers know this, they know that this imam sits for the sitting of rest, they will not rush, they will sit with him, they will follow him, and this is the original way. So if he sits, they sit with him for the sitting of rest, then he stands up.

And if he fears that they might stand up before him, he says the takbir after he sits.

That is, after he rises, while sitting, he says Allahu Akbar, then.

He stands up immediately to the next rak'ah.