Was Mavshibai's 'grinding' massage technique a form of aggression towards Sai Baba?

No, the source material does not portray Mavshibai's massage as aggressive. On the contrary, Chapter 24 describes her as being 'pure of heart in Shri Sai's service' and states that her actions stemmed from 'guileless love.' Although her method was described as twisting and 'grinding' the stomach 'like a pot of buttermilk,' the text explicitly notes that Baba 'did not even say "hay" or "hooy"; he seemed completely at ease.' Sai Baba accepted this 'extraordinary way of pressing' as 'excellent service.' The description was meant to highlight the intensity of her devotion, not any intent to cause harm.