Get into fastboot mode (AOC Q70S15 model) Model: AOC Q70S15
Android version: 4.2.2
Tablet not rooted
Desktop OS: Arch Linux
Issuing the command adb reboot fastboot from my pc makes the tablet shut down but it doesn't reboot, the same happens with adb reboot, nothing change if I do it with sudo.
According to the manual: Press and hold the power button and volume button 3 seconds at the same time. Release the buttons and tablet will boot and an Android robot will be displayed. However 3 seconds doesn't do anything, two scenarios occur when I maintain pressed the buttons for longer time:

After some seconds (10-15) the tablet starts booting but not in fastboot but the normal way, however if I release the tablet buttons it goes off inmediatelly.

Sometimes nothing happens.
Keeping pressed Volume Down + Power for 6 secs did the trick, after that you need to wait like 5 secs and the android robot should appear, then you press the Power button and without releasing it press the Volume Up button one time, now you are the system recovery.