how can I get into download mode of Cube Talk7x (U51GT-W)? I tested all 4 methods in this article. But I still cannot get into download mode.

The second method get me into a factory mode:


  SECOND METHOD
  
  
  Power phone off
  Plug USB into Computer
  Plug microUSB into phone
  Hold pwr + vol up + vol down
  When Batt icon goes away, release pwr, still holding vol up + vol down
  Yellow /_\ (triangle, Downloading, Andy shoveling, Do not turn off Target!!! symbol)
  
  
  You should now be in Download mode.


Edit:
turns out there is no such mode. The adb shell default is root shell. To root the device, I can simply adb pull &lt;system partition&gt;, then mount on PC to add su and other stuff, then just flash it back.
Download mode is a Samsung exclusive partition that only exists on these devices to allow Manual ROM installations from The ODIN PC client. Other devices do not use the download mode you are talking about, including the device you are using this method on.

To elaborate,

ODINs a samsung-only partition that allows form the installation of ROMS and bootloader updates via PC. It can serve Installations to many, many devices at once via bulk download (Which can support 50 at a time, I think. Of course, you probably do not have 50 USB ports) It is primarily used by Samsung when flashing new ROMS for shipping (the software was made by Samsung, after all) and Samsung users who want to root their phones and tablets in a "technical" way (even though the hard work is done by the the Root package Devs, mainly Yabbot, who is better known as Chainfire)