A hacksaw is a fine-toothed saw, originally and mostly made for cutting metal. When finished using a hacksaw, you should place a tension on the blade bending it around the pulleys. You should always relieve tension to prevent long term curvature in the blades. It causes them to not be even in the cutting plane. Over tightening the blade will shorten the life of the blade wheel rubber bearings and the blade.
Detensioning the blade prevents uneven stress on the blade, which can lead to breakage and vibration. It is essential to remove the tension at the end of each work session. You can prolong the quality of the hacksaw by removing unnecessary tension.