The phrase
"flour streaks" refers to lines or marks made by flour, often visible on something that has been coated or covered with it.
Full definition
Beat just until blended and
no flour streaks are visible.
Add to flour mixture and stir gently, just until ingredients are moistened and
no flour streaks remain.
Mix in dry ingredients until
no flour streaks remain.
No flour streaks should be visible, and the nuts should be evenly distributed.
Place the mashed potato in a bowl and add half the flour to it, kneed with your hands until
no flour streaks remain, then add in the rest of the flour, again kneading until mixed through.
Pour wet ingredients into dry ingredients and whisk until just combined and
no flour streaks remain.