[range.chunk.fwd.iter, range.stride.iterator] What precondition is implied here?

From [range.chunk.fwd.iter-12] / [range.stride.iterator-13]:

constexpr iterator& operator+=(difference_type n) requires random_­access_­range<Base>;

Preconditions: If n is positive, ranges​::​distance(current_­, end_­) > stride_­ * (n - 1) is true.
[Note 1: If n is negative, the Effects paragraph implies a precondition. — end note]

Effects: Equivalent to:

if (x > 0) {
  missing_ = ranges::advance(current_, n_ * x, end_);
} else if (x < 0) {
  ranges::advance(current_, n_ * x + missing_);
  missing_ = 0;
}
return *this;

Maybe it's my English comprehension issue, "If x is negative, the Effects paragraph implies a precondition." What does this mean? What precondition is implied here?

Start with range.chunk.fwd.iter-12 and range.stride.iterator-13, especially the Preconditions and Effects for operator+=. Determine the precondition implied by the negative-n branch and clarify the standard wording so the requirement is explicit; it is done when the interpretation and any needed editorial change are agreed.