The separation simulated by this test will take place after the first stage solid motor completes its burn and will be effected by a ring shaped explosive charge that cuts through the aluminium structure that joins the two stages together. Remember, this structure must be thick enough to transmit the high bending loads anticipated during the flight of this long slender rocket, called the "Stick" by some.
The test shown in this slow motion video was to check that the separation charge makes a clean break between the two stages. The shocks generated by the explosive charge were also measured during the test, and this will give important information enabling the engineers to design and layout the nearby equipment mounted in the launcher that must withstand the shock environment.
Earlier Post: When the Change the Ares-1 "Stick" into a Normal Rocket