A technical t-shirt uses heat-activated adhesive tape bonded to panels with an iron or steam, then reinforced with a tape seaming machine. Bonding alone won't survive repeated washing, so the seam lock adds durability that sewn hems provide in regular shirts.
So what she's doing right now is called bonding. This piece of white paper has adhesive under it, and in order for that adhesive to be activated, it needs to be heated up. So as she removes it and folds it over, the iron steam heats up that adhesive and allows it to glue onto the panel itself. So for most t-shirts, it's sewn on the hem of the sleeve. In this particular case, this is a more technical t-shirt for a more technical brand. They actually just do it with bonding. In reality, that adhesive is not strong enough to hold over time for all the washes that the garment is going to go through. You need something that's really going to lock it into place, and that's what this tape seaming machine is used for.
