Modding Help Homing Projectiles

I know that some projectiles are homing (like the Shockhopper missile), but trying to use these against monsters has no effect. A few mods have achieved this, so I know it is possible (the Gjallarhorn from FU and the Ancient Bow from Extended Story are good examples).

Does it have to do with the controlForce parameter in the projectile file, or is there more to it than just tweaking numbers?