This command sets the URF (some German: Unterer Relaxations Faktor)
for the fit. This value determines who 'fast' the fit will move to
its minimum or how much the parameter values are changed in each
cycle depending on the deviations. A small value (e.g. 0.1) might
lead to a fast convergence but might also miss the minimum. A larger
value (e.g. 100.0) will give a slow convergence which more certain
finds the minimum, but might be caught in local minima rather than in
the global one.
Understood ? Well just try different values until your fit converges
nicely to the global minimum.