It will give you a profile from your position abeam the extended center line as if you were established on it. Because VNAV is always calculated from track miles off the magenta line it uses track miles to go on the line from your abeam position (as if you were on it so to speak).
An example: If you were turning base towards the extended line (90 degree intercept) and had around 5 track miles to run before intercept, it would show you as around 1500ft high if you were spot on profile. As you approach the line the pointer will count down and come nicely onto profile.
This works well if you turn outside the active way point.
If ATC vectors you inside the active way point then it will calculate your abeam position to the active way point and then back again.
I normally just bring the final fix to the top and intercept off that.
Another way of getting a good profile without the mental calc is to select LNAV when on an intercept, and it will give you the correct profile straight away as it now knows how many track miles you have to run.
Hope that helps a little.
