The jumping digits to letters transformation
Python 3, 60



i=b=0
for c in input():b and print(chr(97+i%26));b^=c&lt;1;i+=c


Takes a list of numbers and prints the chars on separate lines.

Basically just follows the instructions. The index is i and the should_write flag is b. The expression b and short-circuits to print only when b is true.