This variable defines whether or not the instance is extremely
fast moving (for example a bullet). The default value is
false but if set to true this tells GameMaker:
Studio that the instance will be moving at such high speeds
that it will require more expensive collision detection to ensure
it doesn't pass through other instances undetected.
with (instance_create(x, y, obj_Shoot))
phy_bullet = true;
physics_apply_local_impulse(0, 10, 0, 200);
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.