During curl ups, the trunk flexors exert less force if the arms are thrown forward during the trunk movement because of

Of the transfer of momentum

Well, when your arms are put forward, they do help you very much on doing the curl ups. It doesn't make anything easier, it just makes it to the point that you're not as uncomfortable. The reason why the exert less force is because of the transfer of momentum. There would not be enough momentum to help you get through that lift if your hands are put forward.