If the pipe is not to be disturbed then, the Host application will need to "Flush" the IN buffer until it NAKs. Therefore, if the Host application tells the firmware to clear the buffer, the Host will need to perform INs until NAKs are received prior to the firmware placing a new set of packets into the buffer.
However, if a USB Reset has occurred or and alternate interface is selected, the endpoint can be cleared of its previously armed packet by writing a '1' to the INxCS INxBSY bit to clear the bit. If the endpoint is paired, then each endpoint in the pair must have its busy bit cleared. Additionally, if an application specific set of commands can be used, then vendor requests and acknowledges may be programmed to allow the firmware to perform the reset sequence.