When a car's chassis hits the track surface, sometimes making sparks fly. This is because the suspension has hit the maximum amount of compression it can take, for example due to increased downforce.
This is when the chassis actually touches the ground as the suspension compresses from aerodynamic downforce, and from vertical G forces on oval track banking or over bumps in the pavement.
When the front or rear suspension systems use ALL available travel to absorb an impact. Forks or shock have completely collapsed.