OK, I found the stroke when Tatjana hit Darcia's neck the first time. It was actually stroke no. 2.
Darcia was in quite a defiant mood. Now I remember I almost stopped the scene, as the girls started to yell at each others... but then Tatjana's determination restored the order
PS1: It is funny to see that when Tatjana obviously recognized that she accidentally hit Darcia's neck, she starts examining her fingernails, like if nothing special had happened.... It's such a girly thing...
PS2: I don't really see rational explanation behind Darcia's attitude. My strategy in her place would be to generate some kind of sorrow towards me in the dominas, hoping that they whip a little lighter... This defiant attitude however is just not too wise...