Compression: Setting threshold to emphasize body/attack

[bigdaveo11]

I am currently reading Mixing Audio: Concepts, Practices and Tools by Roey Izhaki (Great book with audio examples if you guys haven't already heard of it) Anyway he was talking about setting the threshold level for compression depending on what you are trying to bring out in the sound. For example, in order to bring out the attack the threshold level needs to be set below the body. I do not completely understand what he means by this or how to determine the actual "body" of the sound...(spectrum analyzer?). I assume by setting the level below the body, the compressor will squish the body portion, bringing more clarity to the attack.Vice Versa he mentions setting the threshold above the body to bring out the body, I assume by squishing the attack? To be more general, how do you go about setting your threshold depending on the what sound your after as well as determining the body portion of your sound. I know I should go by what sounds good but these statement in the book were of interest to me, wanted to see what others had to say.

Thank you for your time, I appreciate any responses.