Based on the example from the docs:

>>> it = seekable((str(n) for n in range(20)))
>>> it.seek(10)
>>> next(it)
'10'
>>> it.relative_seek(-2)  # Seeking relative to the current position
>>> next(it)
'9'

At this point, I would expect the following:

>>> it.relative_seek(-2)
>>> next(it)
'8'

However, the actual return value is '9'.
The reason for that is that, as far as I understand, the method is not actually seeking relative to the current position, but to the end of internal buffer.