I been reading about RK4 for physics implementation in a game, so I read in some pages and all people recommend me this page:

http://gafferongames.com/game-physics/fix-your-timestep/

This page shows clearly how this one works, but I can't figure out how to implement in my game, maybe I don't understand that good but I find some things that are not really clearly to me.

In my game, the player decides when change direction in the X-Axis but I can't figure out how with this RK4 implementation change the direction of the object, in the example the point goes side to side but I don't understand how I can control when he goes right or left.

So if anyone can give a little bit of clarity in this implementation and my problem which I do not understand I will be really grateful.

Thanks beforehand