Here's an explaination straight from Randy Horton.
"A feature unique to the 705 is Tracking Ground Balance Offset. In simple terms, this allows the user to run in Tracking and at the same time, maintain a Ground Balance setting that varies from the one “recommended” by the electronics of the X-TERRA.
On the 705, you can adjust this offset to be anywhere from +1 to +15 or from -1 to -15. There has been some discussion as to whether lowering the ground phase number makes the GB of the X-TERRA more positive or more negative. I think the use of the word “Offset” is what makes this confusing. Setting a + number into the Tracking GB Offset will provide a more negative ground phase than the number the detector has chosen as “neutral”. For example, if you are hunting an area in Tracking, and let’s say the X-TERRA has determined at a given time the proper GB setting is 35. That means the 705 software is going to set its ground phase at 35 in an effort to best neutralize the effects of that soil. If you had programmed your 705 with Tracking Ground Balance Offset of +5, your X-TERRA will actually indicate a 40 on the display. In this situation, the 40 that you see on the screen is the sum of what the X-TERRA software has determined to be required (remember it was 35 in this example), plus the +5 Offset that you programmed into it. By setting a +5 in your program, you have told the 705 that you want to run a negative Offset of 5, in combination with what it determines to be the proper setting.
On the other hand, if you want to run a positive Offset, you’ll need to enter a negative number. For example, if you want to track with a positive Offset of three, you would need to enter a -3 into the procedure. If, as in the situation above, the 705 determines that the perfect setting would be 35, your positive GB offset of -3 would actually make the display indicate a 32. Personally, in most of the sites I hunt, I like to run my X-TERRA with a slight positive Ground Balance (ground phase setting)."
I hope this helps you to understand how it works.