Answer
Because the tendon wraps around, the distance between the point where the force is applied and the axis of rotation is larger. Then the torque is also larger which makes it easier to rotate the lower part of the arm.
Work Step by Step
The torque is proportional to the distance between the point where the force is applied and the axis of rotation.
If the tendon did not wrap around, then the distance between the point where the force is applied and the axis of rotation would be very small. Then the torque would also be small which would make it harder to rotate the lower part of the arm.
Because the tendon wraps around, the distance between the point where the force is applied and the axis of rotation is larger. Then the torque is also larger which makes it easier to rotate the lower part of the arm.
