In order for a monster truck to be able to do a front flip it needs for one of its points far from it's center of mass to come into contact with a fixed object. One way of doing this would be to get the truck into a wheelie or almost stand the truck up, then you need to hit a fixed object like a very steep jump. This will cause for the truck to begin a rotational motion and begin to do a front flip. This is because the impact of the tires on the fixed surface act as a torquing force which cause rotational motion. In the video below the truck hits a jump to get the car into a wheelie and then increases speed to hold the wheelie so that the trucks back tires come into contact with a dirt ramp which cases the truck to perform a front flip.