what is the pilot injection? and what is it used for?

It is fuel that is injected before the main injection to help reduce engine noise.

It also turns off above 2300 rpm or so, you may notice your engine get a little louder at that point.

So is this the part that makes a change in sound when shifting under load?

It is fuel that is injected before the main injection to help reduce engine noise.

Exactly. And it works good on a Duramax to keep down the noise..

I wonder if it wastes fuel? and why it shuts down at 2300 RPM? that's about 70. It's not noticeable unless I'm running a hot tune then... rattle rattle rattle at about 70-75 MPH.

http://www.duramaxdiesel.com/02_bosch/sub03.htm

The following info copied from link at bottem, which was used to install ODLO on the 01/02 models.

Diesel engine fuel injection systems that use a common rail system, as the Duramax™ diesel does, may or may not have what is referred to as pilot fuel injection at the lower engine speeds. Pilot fuel injection is defined as a pre-injection of a small quantity of fuel 5 to 20 crankshaft degrees before the main fuel injection. This pre-injection softens the main injection pressure rise and reduces combustion noise. At engine speeds above 2600 RPM, and a fuel quantity supply greater than 35 cu mm, it is necessary to eliminate pilot injection to meet emission requirements. When driving at or above these speeds, main injection combustion noise may be noticeable. It is normal and not harmful to the engine.

When traveling in 5th gear, the pilot injection to main injection transition is typically in the 136 km/h (85 mph) range, however it may also be heard under heavy acceleration and when pulling loads up a hill during shifts at or above 2600 RPM. Customers that install the fifth gear inhibit switch may encounter the fuel knocking of main fuel injection at speeds around 107 km/h (67 mph) in fourth gear between 2600 and 2700 RPM. This sound is considered normal and is to be expected.

http://dieselplace.com/forum/showthread.php?t=708