It is maybe first chapter of the "Kaisou,Haha no syoujyo jidai"(lit. Look back upon Mother's Girlhood) .
(Japanese site on this)
While Tubaki spanks shiho and shiho is crying,she just recalls she herself was spanked by her mom when she was young.
After this movie,young Tubaki is spanked by her mom on bare bottom with ruler and Miyazaki Shiho plays young Tubaki and KANDA Tubaki does her mom,so we can enjoy Shiho and Tubaki's spanking enough.
If you get full movie,i recommend you search the Internet with the keywords; for example m_mom2_r01.wmv,m_mom2_r02.wmv,m_mom2_r03.wmv...