Modifying counter within a loop

[Fritz Sonnichsen]

I have a stretch of code that contains a "for" loop. Within the loop, 
under certain circumstances,  it adds a constant to the loop counter for 
various reasons. This allows the code to "skip ahead". However after 
some investigation it is apparent that one cannot do this--that is, the 
loop counter is always set to the next consecutive value and ignores the 
added constant.

I assume this is a rule for the loop command (could not find this in the 
docs). Is there a way to override this behaviour?

Thanks
Fritz