How does the combination sum problem use backtracking?

Start with the first candidate number and a remaining target. At each step, either include the current candidate, subtracting its value from the remaining target and staying at the same index since reuse is allowed, or skip it and move to the next candidate. If the remaining target reaches zero, record the current combination as valid. If it goes negative or all candidates are exhausted, backtrack. To avoid duplicate combinations in a different order, only consider candidates at the current index or later, never earlier ones.