Do you remember the lift? When the lift is greater than the weight of the glider, the nose of the glider moves (pitches) upwards. Then, when the thrust is created using the pops connected to the glider, it pulls the glider to an increasingly steep angle. The amount of lift actually decreases as more force comes from the pull of the upwards. When the glider is completely vertical in the hover position, there is no lift on the wing of the glider. So, the climb is nothing but the front/nose of the glider moving upwards. The angle is also known as the Angle of Attack (AOA).