From one of our veggie specialists:

Many of us are already planting, or contemplating planting, indoor starts for our vegetable gardens. We give our indoor starts strong overhead lighting, warmth, and adequate moisture to promote strong early growth. But did you know that your seedling plants also need exercise? This is especially important for those noodly stemmed baby coles (cabbage, broccoli, etc.). Gently bending them every day, called ‘brushing’, will cause the stems to thicken and the internodes (distances between each set of leaves) to shorten, creating a stronger seedling with more resistance to wilting. Outside, this would be accomplished by wind. You can do this daily by gently bending your seedlings to almost horizontal, without breaking them, with the side of your hand or a piece of cardboard.