I know the feeling, happened to me once while instructing in 206. The guy had about 30 hrs in turbine and I would say that in every single flight lesson I told him to stay on the starter ītill 58%Ng.
Well this time he let go, but at about 30%. Seems she was already under self sustained start/run by then because the temp didnīt really seem to increase any for the fraction of the second it took me to hit the start button. I always keep my right hand on the collective, with my thumb resting on the collective switch panel (can you say that for B206, not that many switches there!) when instructing.
Sure was scary, but what impressed me was that there was no apparent temp rise.
Bendix, gotta love it : )