On a properly tensioned wheel, all spokes are under tension at all times. Imagine the bike at rest, with a single spoke for each wheel, and the hub is suspended from the top of the rim by that single spoke. Those two spokes will be supporting the weight of the bike and rider. Now add the rest of the spokes and tighten them all up. As the wheels roll, the tension in the spokes in the upper half of the wheel increases to support the bike and rider and the tension in the bottom half of the wheel is reduced by the same amount. All of the spokes must be tight enough to stay in tension at all times. If the spokes are not tight enough, as the wheels turn, the spokes in the bottom half of the wheel can become loose or go into compression, which can lead to either the nipples loosening, or the spokes "flexing" and breaking, or both.