It's likely something similar to how mechanical broadheads work. The blades are situated so that in flight they are held closed with a small plastic retaining r

It's likely something similar to how mechanical broadheads work. The blades are situated so that in flight they are held closed with a small plastic retaining ring but on impact with the target the blades cam open. It's very low-tech but very effective. Here's some slow-mo shots of mechanicals in action: https://youtu.be/6dv1QySWQ6I https://youtu.be/6dv1QySWQ6I https://youtu.be/PZjEhk-CkKc https://youtu.be/PZjEhk-CkKc