Once upon a time, back in the olden days of F1, there were no restrictions on the ride height of the car (except for suspension travel bottoming out the underside of the chassis). When the car is lower to the track surface, the air underneath moves faster and faster, sucking the car onto the ground. What you are seeing is the back of the chassis hitting the ground over the bumps into the braking zone on turn 1.
In 1994, the FIA introduced a 'plank', a piece of wood that is mounted underneath the car to stop the cars from getting too low. It is 10mm thick, and if it less than 9mm thick in any spot, the car is illegal. To get around this ruling, the teams have legally covered the whole plank in titanium screw heads.
That particular shot was the result of Mansell chasing down Senna for three laps and not being able to get past until that moment.
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