There are 3 typical responses to |27.q20: 28.p20, 28.28.p22, or 28.r20. In this game, I believe 2 of these are winning for Black, and 1 of these is losing. And Black chose the losing one, as shown by ptolemycasio above.
Then White handed the victory back to Black with |29.w21. All Black had to do was 30.t19.
Then White handed the victory back to Black with |29.w21. All Black had to do was 30.t19.
After that, I believe White was winning.
White would have had a cleaner victory with |35.t7. 36.s10 37.p9 shows Black can't leave a gap. 36.r8 37.u9 38.s10 39.t12 or 36.q9 37.u9 38.r11 39.t12 would end it quickly.