A couple of days back I got the book Among the Elephants - Iain & Oria Douglas-Hamilton. He was the first to do systematic research on elephants in Africa.

I found that he has also mentioned this behaviour in this book.

Among the Elephants – Iain & Oria Douglas-Hamilton
Page 68, 4th para
"Another distinctinct pattern of behaviour with practical as well as theoretical interest was the twiddling of the trunk, the swinging of one of the front legs to and fro, and rocking from side to side which I saw when an elephant appeared to be deciding between attack an retreat. These were typical ‘displacement activities’. In elephants they were a great help to me in predicting their behaviour. The more marked these activities the less likely the elephant was to charge. Very often the most impressive threat displays emanated from the most frightened elephants which were unlikely to make a serious attack.

Niko was particularly interested in the differences in character between individual elephants where this led to their behaviour becoming predictable. As we passed one matriarch named Inkosikaas, with an upswept tusk like a sabre, she shook her head in mild annoyance. I stopped the car and told him to watch these elephants, because within about five minutes they would charge. Inkosiakaas fiddled with her trunk, then turned to the other cows on either side of her and clashed tusks with them in turn, putting her trunk in their mouths on e after the other. This seemed to reassure her and almost exactly five minutes later she delivered a beautifully impressive threat charge. This was her own predictable quirk, unique among the elephants of Manyara: the tendency for delayed action aggressive displays."

The learning apart from the elephant behaviour is that, we have failed to tap the knowledge. I first read this from a hunters book written in Oriya. I read it in the late 70's and saw the behaviour in 90's. This hunter had experienced this behaviour in the 1930's and 1940s. Douglas-Hamilton recorded this behaviour in African elephants in the late 1960's and early 1970s. Still this is not known to people. I hope we can collect all such knowledge and bring it infront of the world.

Cheers,
Sabyasachi