Well, he had a vocabulary of 50 - which grew out of the 5 base vowels, of which we're really just talking about two. Anyway, Kototama can get quite complex, and gets into much more tangled knots than just pressurization (I'm not saying that pressurization isn't important - although I think it can be quite tricky to actually pressurize correctly).
In Mashilo Nakazono's book "Inochi" the book of life, he talks about three principles represented by different orders of the Mother sounds (vowels) Amatu-Kanagi is the first order of AIUEO and represents the material and physical realm of existence. AMATU-SUGASO is the second order of AOUEI and represented the spiritual realm. AMATU-FUTONOROTI is the third order of AIEOU and represents the true realm of existence that combines and brings together as one the material and spiritual. Much like the Kojiki , this book is also very esoteric and goes into great depths of the relationship and order of the child sounds to represent various aspects of life and its creation.
I personally chant the 50 sounds in the FUTONORITO order of AIEOU simply for the physical aspects of the vibrations that are set up in the body while chanting; similar to what Lee mentioned. Interesting thing is that each sound has a certain pitch/frequency (as Marc stated) and will manifest itself in a different part of the body based on the pitch. I have found that the AIEOU order naturally starts higher in the body and goes lower as it progress into the order - this apparently correlates to the higher pitch of A being the highest level and U the lowest level; which resonates very nicely in the center and dantien area. Anyway, with the chanting I use mental intent to follow the vibrations and try to mentally control what is going on in that part of the body at the time. I find it helps in exercising intent as well as getting to know the internal aspects of the body for IS development.