Holding a dumbbell overhead with arms locked out while curling the torso forward into a crunch lengthens the lever arm the abs have to move against. The straight-arm position means the weight stays far from the hips the whole rep, so even a light dumbbell feels heavier than expected.
Log your dumbbell straight arm crunch sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.