You can think like that:
with f[0:5], 0 is the start position and 5-1 the end position.
with f[0:5:-1], 5 is the start position and 0+1 the end position.
In slicing the start position must be lower than the end position.
When this is not the case the result is an empty string. Thus f[0:5:-1] returns an empty string.
