GPU accelerated OpenCV functions by OpenCV4Tegra with Jetpack installation

My question is how to use the benefit of GPU acceleration when write OpenCV functions on a Jetson TX2 with Jetpack installation.

I have install Jetpack SDK as NVIDIA instruction said. I then wonder when I wrote an ordinary OpenCV function (except SIFT or SURF functions)
do this then run there OpenCV function with GPU under the hood on the Jetson TX2 or do it run everything only on CPU by default.
Is there any special treatment to select the GPU enhancement or do it run GPU automatic because of the OpenCV4Tegra installation ??

Best Regards
Olle Welin

Please refer to the update in https://devtalk.nvidia.com/default/topic/1030872/jetson-tx2/compile-on-jetson-tx2-a-opencv-example-with_cuda-on/post/5245382/#5245382

Thanks

Thanks