The sequence goes something like this:

Huygens deploys first parachute.

Heatshield bolts are severed.

Probe has a parachute to slow it's descent, so it floats down to the surface.

Heatshield (which is now seperate from the Probe) does not have a parachute, so it plummets at a signifigantly greater rate, "like a rock".

You don't need any fancy high-explosive bolts (or even springs) to accomplish this, as gravity and aerodynamics will solve the problem for you.

Here's a good math problem: How many kilograms of TNT would be needed to send the huygens probe flying 60 meters into the air on titan? 30 meters? 30 feet? What if they used C4?

__________________
audentes fortuna iuvat