upper part of some boots on the outside back of the heel, narrowing and extending upward over the ankle
Jockey backstraps are particularly common in patterns with galoshes, where they shorten the large and often inefficiently clicked shape of the galosh parts, which no longer need extend back to meet at the backseams.
a broad backstay extending forward and meeting the quarter on each side
— The Shoe and Leather Lexicon, Fourth $Edition