You log a set between sets. Standing up, chalk on your hands, forty seconds before the next one owns you again. That is the entire context in which this screen is ever used, and for too long the screen behaved as though you were seated at a desk with time on your hands. Nobody using it has ever been seated at a desk with time on their hands.

What changed

Prefilled working weight. Each set row opens with the weight and reps you used last time on that exercise, so a matched set is one tap. The app remembers so you don't have to; remembering being the one job an app can actually be trusted with.
The rest timer starts itself. Finish a set and it's running. No second tap. No discovering, three exercises later, that you have been "resting" for eleven minutes.
A keyboard that knows its place. The exercise and its previous sets stay visible while you type, instead of the keyboard rising up over exactly the numbers you opened the app to see.
Undo. Tap the wrong row and you get five seconds to take it back, because you will tap the wrong row. Everyone taps the wrong row. Design for the person you are, not the person the mockup assumed.

Why it matters

Most tracking apps lose you at the point of entry. If logging outlasts the rest period, you stop logging (not out of weakness, out of arithmetic), and a log with holes in it is worth almost nothing twelve weeks later, when the holes are precisely where the answer was. Every change above exists to shrink the gap between finishing a set and having it written down, because that gap is where tracking habits go to die.

Nothing about your history changed. Old workouts are untouched, and the prefill reads straight from them. Your past self, it turns out, was taking notes for you all along.