After the tensioner's set and assuming you've got the bar nuts at about 1/3~5/8 of a trun loose.
One hand holds the rear handle and the tend region/tip of the bar is resting on the wood
{You do have some wood handy doncha? }
Then while keeping this araingment, use your rt hand to place the scrench
on a bar nut, with the screwdriver tip pointing a bit upwards and towards the tip of the bar. Now snug the nut and repete on 2nd nut. Viola!
your bar's in the upward position and the nuts are tight.
just double check your chain tension afterwards.
Genraly, with a bit of practice, you'll find that the little chain getting a hair tighter after tightneing the nuts, gets a lot less of a problem.
but kepping the nuts snug-- Not tight-- while you set the tensioner it a big key to reducing that little agrivation of the tension shifting.
And remember, the tensioner is NOT a bar holder, the nuts do that job.