he put his hand in Fenrir's mouth to distract him so that he could be put back on his "leash". only by sacrificing his hand were they able to regain control of Fenrir. so you can look at it as foolish but i think it was pretty damn brave and shows how Tyr put his people's well-being above his own.
Nice. Tyr is not a very popular subject in modern Norse mythological art, but this is an excellent depiction. The symmetrical pose highlights his lawful nature, and attaching a flail to his stump was a genius idea. plus they accuracy is nice - sometimes people like to draw him without his LEFT hand, which is horribly wrong. his RIGHT hand was torn off. that's actually pretty important.