Quoting Chony,
"I never use style touch, in fact I'm not even sure what the purpose of it is... Why would you want each chord to be played at a different volume (including drums). I'm curious which style of music that is good for?"

Style touch is a means of introducing some velocity sensitivity to the style parts- but not drums. They rise and fall together. You can keep the level constant or rise to crescendos or maintain a higher level for a few bars depending on the expression you want and the way you play. The sensitivity is not normally as great as is usual with RH voices.
I thought the Ty3 (and prior) styles just had 2 levels (normal and high) but looking at Ty4 there seems to be a more continuous transition between normal and high.
Perhaps a change has been made, and one of the reasons I thought it worth checking.

john