Nikon say, The AF-ON button is parlucitarly useful for sports photographers to focus track a subject before shooting with the shutter release. Using this method restricts the time the VR system has to activate and therefore sometimes it does not have time to fully stabilise the image. If VR stabilisation is an important consideration it is recommended to always use the shutter release to ensure that the VR system is activated during focus tracking. Worth knowing also, when shooting without a tripod, using a VR lens.
Nikon say, The AF-ON button is parlucitarly useful for sports photographers to focus track a subject before shooting with the shutter release. Using this method restricts the time the VR system has to activate and therefore sometimes it does not have time to fully stabilise the image. If VR stabilisation is an important consideration it is recommended to always use the shutter release to ensure that the VR system is activated during focus tracking. Worth knowing also, when shooting without a tripod, using a VR lens.