The stages of the Martlet 1 rocket fit together with some finely machined aluminium coupling rings. The coupling rings are then restrained together by some 6mm plastic bolts. In flight, the primary method of separating the stages is to ignite the rocket motor in the next stage. As the pressure in the rocket motor ramps up, the high pressure causes the plastic bolts to shear, and the stages separate. The stage that has separated then deploys its parachutes from the open end, by using explosives protractors to release the spring drogue parachute.
If the rocket motor fails to ignite, however, it is important to have a secondary separation mechanism, so that the parachutes can be deployed. In this case, the flight computers detect that the motor didn’t ignite, as there is no acceleration, and separate the stages. The explosive protractors that deploy the spring drogue parachutes are strong enough to shear the interstage plastic bolts, and the stages separate, with the spring parachute close behind.
This video was taken at 300 frames per second and shows the front of one stage (bottom tube) and rear of the next stage (top tube) separating. As the protractors are activated the cap that restrains the spring parachute pushes the stages apart with 4kN force applied within 20ms of activating. The plastic bolts, whose heads are the white dots, are sheared and the orange and black spring parachute jumps out of the parchute section. In this video the spring parachute does not jump fully clear, as (a) the spring was not fully compressed, and (b) the bottom tube hits the floor before it can get out.