Pitch stability on a plank, (flying wing), is maintained by creating a second, (negative), centre of lift on the wing using the upturning of the wing trailing edge, (reflex).
The Balance point of the wing is shifted forward and the reflex supplies a counterbalancing force against the wings tendency to want to pitch downwards with the CG forward of the point of maximum lift.
The amount of reflex required is dependant upon the wing loading and the balance point/CG point selection.
Whilst stability can easily be obtained, the short coupled nature of the plank means that a degree of precision is required in setup and in addition the wing will be very sensitive in pitch to control inputs. Do not forget the Expo! One other thing to remember is that planks, unlike other flying wing types, really benefit from adjusting the control mix ratios in the elevon mixing.
The plank will fly with a large range of setups though there is an optimum setting which can be determined during the initial test flights.
I hope that helps
Have fun!