Answer:

An algorithm is incremental if it does not need to recompute everything after a small change (like a new element is added).  Insertion sort traverses through unchanged element, stores the new element into a temporary variable and loop from right to left to find the correct position. The new element is then inserted to the list. Insertion sort is incremental when the new element is added at the back of the list.

To add on:

Because all original sorted elements would not move as they stop on the first comparison, only when it reaches the new element (last element) does a sequence of swapping and comparing occur, and since it is the last element, the sorting algorithm knows the list is already sorted.