The trap is sprung by insects moving 'trigger' hairs on the lobes of the trap.
There are generally three or four hairs on each half of the trap. The trap is closed by the outer wall of the lobes 'growing'relative to the inner surface. This action is incredibly quick, the trap is effectively closed in under 1/3rd of a second.
For the trap to close the plant must sense that a trigger hair has been touched more than once within about twenty seconds. Similarly touching two trigger hairs also springs the trap. The need for the second touch is generally regarded that the plant is requiring confirmation that there is actually an insect worth catching currently stood within the trap. This requirement is easily understood when you consider that relatively small insects will simply be able to walk out through the spines, thus having caused the plant to expend valuable energy for no reward.
Once the trap is sprung and the spines are interlocked the plant then has an additional check to ensure it has actually caught something. Now the trigger hairs must be moved again, before the trap completes its closing phase and hermitetically seals itself, and digests its prey.
The activation of the trigger hairs after the trap has been sprung is required to indicate that an insect has been caught and the hairs have not been triggered by an inert object such as a leaf or other wind borne debris.