Why do Indian trucks say "Horn OK Please"?

The first theory sounds plausible to me, but it is not explained clearly in the text.

"Large trucks had limited rear visibility" => The main problem when overtaking is that the truck who wants to overtake, has limited front visibility of oncoming traffic.

So who has to honk? The front truck, who sees that a truck behind him wants to overtake, honks to signal that there is no oncoming traffic? Or the rear truck, to signal to the front truck that he wants to overtake? And then what?

Or is there some implied sequence of honking: first the truck that wants to overtake, to signal its request, and then the truck in front, to signal that there is no oncoming traffic?