Wave-Particle Duality Laterally Programmatically
C, 69 bytes

p;f(char*s){p=s[strlen(s)-1]^46;p^=p?93:3022856;printf("%s%s",s,&amp;p);}


This requires a little-endian machine, and output to a terminal that support ASCII escape codes.

p=s[strlen(s)-1]^46 grabs the last ASCII code of the input string, and XORs it with the ASCII code of a dot.

p^=p?93:3022856 will cause p to be p^93 if the ASCII code is not a (back)slash, where p^46^93 == p^115, which will toggle between back and forward slash. If p is a dot, it will instead be 3022856, which is little-endian for "\b .".

printf("%s%s",s,&amp;p); prints the input string followed by the integer p, interpreted as a little-endian byte string.