NVIDIA's nvflash program is used for controlling/manipulating a device in APX mode. Thus APX mode is sometimes called nvflash mode, and the device is said to be providing nvflash access. APX mode is sometimes also confusingly called recovery mode — APX mode is different and more primitive than a tablet booted to [Android] recovery.
APX mode is triggered by holding some device-specific button combination for a few seconds. APX mode can be triggered at any time: while the device is booting up, while it's running Android, or while it's running Android Recovery.
When a device is in APX mode, and connected via USB to a host PC, the device will identify itself with vendor "NVidia Corp." For the ASUS Eee Pad Transformer, the screen will initially be blank, then show status information as the tablet processes nvflash commands. When the device is running in its normal mode, it should report instead with the device vendor's name, e.g. "ASUSTek Computer, Inc."
To exit APX mode, press and hold the power button for about six seconds. The device should immediately boot into its normal mode.