Sorting Algorithms (bubble sort, merge sort)

What optimization strategy is demonstrated in this hybrid sorting approach?
function hybridSort(arr, threshold = 10) {
  if (arr.length <= threshold) {
    return bubbleSort(arr);
  }
  const mid = Math.floor(arr.length / 2);
  const left = hybridSort(arr.slice(0, mid), threshold);
  const right = hybridSort(arr.slice(mid), threshold);
  return merge(left, right);
}
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