Bumblebee doesn’t use nVidia GPU by default. Default Bumblebee behaviour is to always use Intel GPU. It allows to save the power, and to avoid laptop’s overheating. nVidia GPU, under Bumblebee control, must be always switched off. As reported by the optirun --status command:
$ optirun --status
Bumblebee status: Ready (3.2.1). X inactive. Discrete video card is off.
If nVidia GPU remains switched on, it is an abnormal Bumblebee behaviour. This is what is called Missing power off bug in the Wiki.
A command, an application, a program will run on nVidia GPU only when it is explicitly called with optirun prefix. Without optirun it will executed on Intel GPU. An example:
pwd - print working directory command will be executed on Intel GPU
optirun pwd - print working directory command will be executed on nVidia GPU
switches nVidia on upon startup
executes a command on nVidia
switches nVidia off when terminates