She’s not slicing the cake properly. A sawing motion, as you can clearly see in the video, results in icing sticking to the knife. As her knife went up and down, she smeared the icing down the inside of the cake.
The reason she’s using a sawing motion is she’s trying to cut the cake with the tip of the knife. That is also the incorrect way to slice a cake. Sawing motion, in short incremental strokes forces you to break through the crust over and over again. In addition to smearing icing, it causes the cake to crumb a lot more than a swift, single stroke slice.
Her knife is also too short. Most people do not think of the knife length because they either cut the cake with the tip as she did, or more commonly, they begin slicing the cake from the center to the outer edge.
The correct way to slice a round cake regardless of the type of knife:
1. Select a knife longer than the diameter of the cake
2. A straight edge blade should be thin, not thick like a chef knife
3. A serrated edge blade should not have worn scallops
4. Heat the blade by placing it in very hot water for a minute
5. Dry the blade
6. Place the blade edge down fully across the center of the cake
7. With even pressure, slice straight down
8. You will feel initial resistance as the blade cuts through the crust
9. Maintain the same pressure; once through the crust, the knife will easily slice through the soft interior
10. Wipe the blade clean
11. Place it in hot water to reheat; dry and repeat for each slice
The scalloped edge of a serrated blade has fewer points contact than a straight edge blade. But the pressure on the points of contact are far greater. The increased pressure on the points of contact and fewer points of contact results in a cleaner slice. When a cake is very light and airy, a straight edge blade will squish the cake. This principle of fewer points of contact is best demonstrated with the Angel food cake slicer. It’s not a knife; rather, it’s a large fork like comb. The tines ensure the fewest points of contact; you make a clean slice without squishing the cake.
If you are concerned about slicing technique, consider baking a batch of cupcakes to practice on.
One last point about both videos you referenced: caterers and event cake bakers do not slice round cakes in wedges. Wedges dramatically reduce the number of servings. When designing an event cake, event slicing patterns is used to determine the size of the cake. If you are the host, estimating servings based on wedges like that will result in your spending 3-4 times more money for a cake way larger than you need. If you are the event baker, then you need to know how a cake is sliced in order to design and bake a cake for the number of guests.
Just because someone posts something on the internet doesn’t mean it’s correct.