Shoe polish is wax.  Wax melts when hot. Spit on a cloth causes it to have more friction.  Friction causes heat.  Friction causes wax, i.e., polish, to melt and develop a smooth surface.

 

Now, if I could just get paid.

1.    What do I owe you?

 

2.    Ergo, if you heat shoes slighty - 110 degrees - it may produce a better shine; unless the shine itself results from the temporary melting and quick resolidification of the wax. 

 

C