Complete all the Thieves Guild quests until you get to the last one. The Gray Fox gives you the boots for an unknown reason. The reason is to fall down a fireplace without dying. If you are a high enough level or have other strong boots, you will not die from this fall. Just as you are falling, take off the boots and put on your best pair. If you wear the boots of Springheel Jak and land, they will break and become unrepairable. If you are strong enough, you will not die from the fall and can keep these incredible boots, which increase your acrobatics by 50.