Temperature smoothing has effect on calculating the low temperature level (i.e. the lower coverline before ovulation).
It can be enabled in: menu -> Preferences -> Method Preferences -> Temperature smoothing.
The rule defines that the 2 highest temperatures (of the 6 previous temperatures before ovulation) can be lowered to the 3rd highest temperature but not lower than the average of these 6 values.
The main purpose of this rule is to ignore high temperatures of possible measuring errors.