There is an added dividend to configuring the AE-AF Lock button to AF-On. When you tap the focus button and release, the lens quickly behaves in AF-S mode. You get instant focus , but then can quickly reframe the scene if you wish.
But, if you push and hold the focus button, you can AF-C, continuous focus that will track moving scenes. In typing out this info I am reminded of the really astounding flexibility and power of this feature. It quickly becomes an automatic reflex. It takes longer to explain than to put into action.