While holding down shift to fast swim/front crawl. When your white stamina bar depletes to empty you still fast swim/front crawl even though you have no stamina left.

Releasing shift returns you to breast stroke and your white stamina bar starts to refill as normal. If you hold down shift, you return to fast swim/front crawl and can continue to swim at this pace without needing any white stamina bar, instead of slow swimming to regen once your stamina has depleted.