To reach Top Safety Pick from the IIHS, cars must be strong enough to withstand the 'rollover' test. This is where weight is placed at one corner of the roof. If the roof gives or caves in in any way under four times the car's weight, it fails. The Ford Fiesta was able to withstand five-times its own weight on one corner, helping it earn the Top Pick status.
Also contributing to the rating was the frontal crash test result. The Ford Fiesta was hurled into the concrete wall at 64.4km/h. The small shell was able to withstand the brutal impact and maintain relative cabin composure. The extent of the impact didn't even reach the windscreen, which normally catches cars out, sending shock waves and buckling the main cabin structure.
Here's the freshly added video anyway. You can clearly see the dummy's head being absorbed and blanketed by the frontal airbag and side curtain airbags in the caption above, even the driver's door remains fixed.