All of the following applies only to gliders, that have a Flap-Aileron
connection, i.e. changing the flap position changes the aileron
position as well.
I can confirm that the ground-runs with negative flaps make a
difference with my Libelle H301. The AoA is the angle between the chord
line and the relative wind. Try this: look at the wing from the tip
towards the fuselage with the stick centered and the flaps full
positive. Draw a mental line from the leading edge to the trailing edge
of the aileron. The airflow (relative wind) will be parallel to the
ground during the ground roll. Now put the flaps up full negative and
repeat the above. The line is now at a shallower angle since the
leading edge is still where it was before but the trailing edge is up.
The thing that really helps to gain roll control on take-off with the
Libelle is to get the tail up and 'flying' asap. That way, the wing is
at a small AoA and the ailerons are effective right away.

Uli Neumann