What does the following C code "look for?" $($i$.$e$.$ What kind of change does this C code detect?$)$

$/*...*/$

unsigned char OldSw, NewSw, ChangedSw;

while $($OldSw $==$ NewSw$)$ NewSw $=$ PTH;

ChangedSw $=($NewSw $^$ OldSw$)$ & OldSw;

OldSw $=$ NewSw;

$/*...*/$