I don't recommend messing with that, but... here's how it works:
When you start After Effects, one of the things that it does is do a quick check of the read and write speed to the folder where you have your disk cache. It uses this measurement to determine roughly how long it will take to write a cached frame to disk and read a cached frame from disk on your system. Then it uses that information to decide whether it's worth caching certain frames. For example, if a frame takes 40 milliseconds to render but 50 milliseconds to read from disk, then there's no point in caching that frame. The proclivity preference is a control by which you can make After Effects favor caching more or less.
All that said, this is not something that we would expect anyone to change under normal circumstances. It was put in more for us during testing. But, if you're curious, go ahead and frob and twiddle away. Just make sure that you note the original value so that you can go back to it.
I don't really feel the need to change anything, but back when he mentioned it, I had just put together a new system with a fast SSD and all the highly reccomended parts to make AE scream. So I tried moving the decimal around, but I can't remember if it was 1.00000 or .100000 to begin with. In any event, I could just reset my AE pref file.
Thank you for describing what goes on there. That's actually quite interesting to me.