Many ultralight backpacks come with frame stays instead of a full backpack frame. These are usually one or two aluminum rods that slot into channels in inside the back of a pack. Their primary benefit is that they’re lighter weight than a full frame, but they can usually be bent to customize the backpack to fit your body shape better than a full manufactured frame. Frame stays also help prevent the collapse of a frameless backpack when you load it up with gear. Called torso collapse, the use of frame stays helps prevent the pack torso length from shortening, which can result in poor load transfer to your hips.
How to Bend Backpack Frame Stays