By closing the fingers, the sensation of the tegatana is diminished thereby losing some of its "usefulness" when doing techniques. If the fingers are closed, the sensation "transfers" to the fingers rather than the tegatana. It takes more "effort" to feel the sensation of the tegatana with closed fingers.
doesn't really matter. it's a matter of training and it will get into the whole internal training stuffs which will get folks all excited for various reasons. however, isn't tegetana mean hand blade, i.e. knife hand? you see a knife with that spread? personally, i liked meat-cleaver hand myself.