I have read this article and as far as I understand the body stance (mainly front leg straight and back leg bend) is for sailing downwind. What about 1-Handed sailing upwind? Which is the right stance? (as far as I know when sailing upwind you need to straight you back leg and bend a little bit the front leg) Or is it not possible to sail 1-Handed when sailing upwind?

Hi,

Sailing 1 handed is much more easier upwind. The more you bear away from the wind the harder is to control the power in the sail.
Going upwind means not necessarily bending the front leg. Yesterday I tried to ride upwind with both straight and bent front leg. Both are working but needs different upper body positioning. The main points are weight forward on the harness, and keep the board flat and load the mastfoot. This gives the stability for going upwind and releasing one or both hands.