Sreten,

Front wheels loads are all radial since it cannot pass any torque through the hub (disregarding side forces). So whether the line of force is upward (downtube under tension) or backward, (downtube is compressed) depends on the height of the bump or point of impact.

Draw a line from the point of impact through the axle and see if it passes forward or behind the lower fork bearing.to see which way it'll torque the head tube.

This same question of strike angle is why larger wheels are preferable on bumpy roads.

BTW- if you've ever roller skated, you know that a small bump about 2cm, high that your bike rolls over easily, is enough to stop your feet dead and through you on your face (if you don't step over it).