Download mode is for flashing a new rom to the phone. But it's more low level then the usual CWM method for this phone. Odin is a program used with Samsung phones like the Prevail which allows you to flash the entire firmware including flashing different basebands (radio firmware) while the phone is in download mode. The Marquee and most other LG phones though do not work. It's sorta of an escape route incase you bricked the phone to the point that even CWM or recovery doesn't come up. (unless you have a Prevail and used Rom Manager on it, then you've managed to brick it so bad that even download mode is broke.
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I don't think anyone has made use of the download mode for this phone yet, so it's kind of useless right now.
The equivalent program to Odin is LGNPST and LGNPST_CT (lab version of LGNPST). Though LGNPST can't be used with the phone in download mode since the phone won't show up in LGNPST when it's in download mode. So some other proprietary program LG made is what they use for the phone. Odin could in theory work with this phone, but someone would need to make a working OPS file for it first.
There is a good chance LGNPST could revive my phone given the features it appears to have, but unfortunately I do not have the DLL file for the LG855 (Boost version of the Marquee), so I can't do anything with it.
Example being that the program appears to have the ability to provision the phone and update the PRI (not to be confused with PRL which is separate). It appears the PRI is the source of my phone's problems as it's 3G radio refuses to connect which prevents normal hands free activation and PRL/Profile updates. I really wish I could find that DLL file.
I contacted LG and they say they can reprogram it for me but they would charge $45 + the time I have to wait for it to ship to them in the mail, then for it to come back. That's IF there isn't a hardware defect. If they flash it and turns out it's a hardware defect, then it's $45 + shipping down the drain.