Each stage of the Martlet 1 rocket has two parachutes: (1) a small sprung drogue parachute to bring the rocket down fast from high altitude (2) a much large maion parachute that is released near the ground to give a soft landing. The small parachute is deployed at apogee, and then near the ground the riser cable from the small parachute is released by a pyrotechnic mechanism. As the small parachute is released it pulls the main parachute out of the rocket body.
The release mechanism is shown below mounted on the bulkhead that sits between the motor and parachute sections of each stage. The pyrotechnic device is a protractor (gold in the photo below), which has an explosive charge and a piston inside it. The release mechanism has two parts, a housing that is bolted to the bulkhead,. and a slider with two 4mm pins. The kevlar riser cable (here just string) is looped around one of the pins until the mechanism is fired.
When the pyrotechnic device is fired, a piston is ejected with 2kN force, pushing out the slider and releasing the cable. The slider is then stopped from flying off by a catching plate (not shown).