Stretch Stretch (のび のび) is a yukkuri exercise, in which they stretch out their bodies, usually vertically, as a way to impress onlookers with their agility, or to reach higher places. Doing so tends to leave them vulnerable to attacks, since all of their effort is required to maintain balance, so they are unable to dodge or flee from attackers.
Stretch Stretch is part of the daily exercises of the koyukkuris, as they playfully exercise their still tender skin by gaining the elasticity needed to move easily and build the strenght needed to live in the wild. A Yukkuri parent may teach its ko a poo poo dance, involving exercises as stretching and rolling around, to teach them to defecate without the need of being licked on their bellies.
A stretching Reimu.
A yukkuri Remilia doing stretch stretch as it wakes up.