Intensive iota

I came across the intensive iota on words like τουτὶ or τουτονὶ. First what does this mean and second what are the rules for this iota usage. I can’t seem to find it anywhere. Reading Greek JACT course has it in the text but does not explain it. example:

τί δὲ τουτὶ τὸ πρᾶγμα;

MEANS: “What is THIS matter?”

How is this declined?

χάριν σοι

Παυλε

See Smyth 333g:

http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0007%3Asmythp%3D333%20e%2C%20D