The company I work for has a 206 start simulator. It's a USB-collective with the twist grip and buttons that hooks up to a computer. The instrument panel is simulated on screen with the same layout as in the actual machine.
The software can be set to simulate different ambient conditions, battery states, as well as force hot starts, hung starts etc.
I do some of the annual recurrent training as well as initial training for people getting endorsed on type. Everyone, regardless of experience (pilots ranging from 900 to over 20,000hrs), gets to do at least ~20 simulated starts, including hot starts etc.
Most of our pilots prefer the thumb-start:
RH thumb on starter
LH thumb on idle release
but some (not all) of the old timers do the middle finger - index finger thing as some have explained in here.
Now I am not going to try to change the habit of someone who has been starting 206s for 35 years, but I will say one thing for sure: When I set the thing to simulate a fast TOT runaway, I see A LOT more slipped fingers and resulting (simulated) hot starts with people that use the index finger method than with those using the thumb method.
Clamping the starter button with your right hand thumb simply puts it in a more secure hold than pushing down on it with your middle finger, especially when you then have to try to hit the idle release with a different finger of the same hand. Therefore reducing the risk of accidentally letting go or slipping off. For me personally, it is easier to clamp the right thumb down and then simply forget about that hand completely until the start is finished.
From what I can see out of a sample size of about 25 pilots, very clearly the reaction time and error rate is noticeably lower with the people that use the RH thumb on the starter. Getting the idle release down and rolling the throttle off with the left hand doesn't seem to be a problem for anyone.
So for anyone new to the 206, we will continue to train them to use their RH thumb on the starter, LH thumb on the Idle Release.