Actually very few, if any, NFL contracts are guaranteed. They are essentially a series a 1 year contracts that the team has options on to renew. That is why the signing bonus is so significant to the player. It gives them money up front and also provides a penalty, by way of salary cap penalties, if the team decides to cut the player early. A good example is what the Eagles are doing to Ownes. They have to pay him for the remainder of this season but will not owe him anymore money if they cut him after the season. He will miss out on a 5 million dollar roster bonus if they cut him before March 1st.