In the event of a forklift truck overturning, under no circumstances should the operator try to jump off the machine. Instead the best advice is to grab something firmly such as the steering wheel and also try to brace feet in the foot well of the operator's compartment. In addition the operator should attempt to lean away from the direction in which the truck is tipping. On this one I speak from personal experience as I tipped a truck over back in the 70s. Not pleasant!
In December 2002, because of the LOLA and PUWER statutes, it became mandatory for all counterbalanced forklift trucks to be fitted with seatbelts and for operators to wear them at all times. Exceptions are made where a risk assessment shows that, in certain circumstances, the truck would be unlikely to fall over. Details of that risk assessment can be found here.