The way I read Edyta's tweet, it's not this coming week but the week after next. Maybe I'm wrong
They started doing the dance off during season 8, however the methods they used differed between season 8 and season 9. In season 8, it was used in just the first 3 weeks, and what the judges did was re-score the couple on the dance they did the night before. So the new scores were added to their already combined total and from there that was determined who went home. For the first elimination, Belinda/Jonathan went up against Steve/Karina; Belinda got 1 point lower from the judges than the night before and she went home as a result of the combined votes. Week 2, Denise/Maks went up against Holly/Dmitry, and were sent packing. Week 3 was a double elimination, so there was no dance off. The last dance off was week 4, and the pairing was between David/Kym and LT/Edyta, and David led a foul mouthed tirade as he got voted out of the ballrom.
Season 9 the DO happened in the middle of the season, beginning week 6. The judges this time voted for who they wanted to stay. The total number of votes got the couples to the bottom 3, but they were discarded and then they had to perform a brand new 30 second routine just for the judges decision and the judges were supposed to use that only to vote on. Also, those 2 weeks there were double eliminations. The couple with the lowest total was sent home, and the next 2 lowest scoring couples competed in the dance off. The first dance off was between Louie/Chelsie and Michael/Anna. In a 2-1 decision, the judges voted for Michael/Anna to stay (Melissa/Mark were also voted out this episode). The following week, Michael/Anna got the boot and Aaron/Karina (despite getting 2 10s the night before) and Mark/Lacey were in the dance off. This was the week Lacey was sick and Anna actually stepped in for the performance on Monday night, but Lacey did the DO with Mark. The judges voted unanimously to save Aaron/Karina. There was actually supposed to be a 3rd week of the DO but because Tom Delay had to withdraw from the competition and they eliminated another couple that week, too, they dropped one of the planned DO weeks.
You have to remember though, Kate has to land in the bottom 2 in the first place. The problem they had on SCD, our British counterpart, was the worst dancer in the competition during it's 6th season was also the most popular. He ended up withdrawing without ever hitting the bottom 2 and leaving his fate in the hands of the judges, who would have sent him packing. So the DO, while a great concept and probably meant to save the best, doesn't always mean the worst is going to go home.