I am using
this, except mine is the 360 version. Substitute R2 and L2 for RT and LT. The button in question is RT. Some emulators and controller test apps call it "axis 5+" (like SDLMAME and Mupen64Plus), others call it "axis 4-" (like PCSX), some call it Z-Rotation (Joystick and Gamepad Tester), and some don't recognize it at all (Kega Fusion). Minimum value is 0, maximum is 255. But since it's a button, it never reaches anything in between those two numbers. Most emulators seem to recognize it just fine, but there are a few like PCSX (and PJ64 in CrossOver) that get screwy.
PCSX recognizes it just like other emulators, but it's the only one that constantly keeps it "held down." I'm pretty sure it's some change with the new DFinput plugin, since the old plugin didn't have this problem. In the meantime I just have to play without using that button.