A dynamic instance is one that is fully simulated within the physics world and this read-only variable will return true if the instance being checked is fully simulated or false if it is not
Boolean (or undefined if the instance is not physics enabled)
dir = point_direction(x, y, other.x, other.y);
physics_apply_impulse(x, y, x + lengthdir_x(100, dir), y + lengthdir_y(100, dir));
The above code creates a new instance and then defines it as being a "bullet" in the physics world before giving it a massive impulse along the y axis.