Blending mode results depend on the color. Multiplying red over black will never yield a particulalrly noticeable difference, it will merely make the red darker and darker the more you reduce opacity. The rest sounds inconclusive. Effects don't just not work, but without some screenshots nobody can tell. This could just as well be hardware acceleration issues with OpenGL, some layerr style you have applied or whatever. There's just too many variables.
Matte Choker will make rounded corners because that's exactly what it is designed to do. If you applied simple choker to a text layer and a very small value change makes the text disappear then the text isn't thick enough. Try a fatter font. As Mylenium said, blend modes based on the value of pixels depend on the color. The max value of red is the max value of red so if you multiply it by the max value of red you still get the max value of red which equals the max value of red, therefore there is no change.
This won't work:
This will but barely:
Setting the bottom layer to something lighter with values like this works better because there is some room to do the math and it is in more than one channel:
But you can see that there's very little edge available and the matte choker is rounding the corners. It does this because all matte chokers work by blurring the edge of the alpha and then adjusting the level or the curve of the blur. You can build your own matte choker simply by applying blur to a layer, adjusting the alpha levels using that layer as an Alpha track matte for your text. See how I've crushed the alpha levels toward white? That is what makes the matte shrink.
I hope this helps you understand the process better.
Im not multiplying red over black. Thats just how it looked after the simple choke. It stretched out the sides and looks black buts its actually just the same colour multiplied.
Yes that helps a lot , thanks. However I still dont get why my shape is stretching out the sides when I simple choke?