So... it could be indicative of thinking, uncertainty, contempt or duplicity.
to be fair, I thought I should rule out habit, or illness as the cause so I reviewed several other interviews and pressers involving the Commissioner.
I could find only one other time he exhibited this same non-verbal language. That was in giving a presser to defend the AFP against charges of selling the Bali 9 up the river.
In the video posted here, look at the
1:25 mark where he is stating when the government knew about the NBN investigation
1:53 mark where he states that relevant parties have at all times been advised of the nature of the investigation
2:02 mark where he says no two investigations (of leaks) are the same*
2:40 mark where he states there was no government influence**
* This statement is true. It should go without saying that no two investigations are the same. But what Colvin was trying to defend here were allegations that some leak investigations get priority while others never see the light of day. There is a difference between "no investigation at all, or a short and shallow one" and one "coinciding with an election and including raids on the government's political opponents". The inference to be drawn from the allegations is that some leaks happen on purpose for the benefit of the government, and will not be investigated - even those involving National Security - while leaks damaging to the government will be vigorously pursued.
** This is an interesting one because he had earlier made a similar statement without the tongue action. So why did it happen on this occasion? Perhaps because in his first statement on this issue he said there was no government influence ON THE INVESTIGATION. This was no doubt true, but says nothing of whether there was government influence to COMMENCE the investigation.
So, was Colvin thinking, showing uncertainty, showing contempt, or being duplicitous, or is there some other explanation? I'll let the readers (and viewers) be the judge.