Knockdown shields can be a pain to deal with, especially if your target keeps scurrying away with a legendary shield. However, a method to bypass enemy knockdown shields has been discovered by Reddit user Cpt. Pugsy that completely nullifies the usefulness of the shield entirely, making for a quick finisher in the heat of battle.
By starting a finisher on a downed enemy and then immediately canceling it, the enemy's knockdown shield will be down for a very brief moment, leaving the enemy vulnerable. With a quick trigger finger and having a weapon that can deal a huge amount of damage in one shot like Peacekeeper are capable of taking out the enemy in this small window.
The utility of this trick is fairly situational, as most of the time it would be more beneficial to complete the entire finisher in order to recharge your shields if there are no direct threats nearby. However, in the heat of battle and you are desperate for new armor or are feeling greedy for a quick elimination its a great tool to pull out that can make the difference between being downed or winning the entire match.
The technique seems to be an unintended consequence of the new finishers that was overlooked by Respawn and it is possible that the trick will be patched out in a later update.