Following on from https://derpibooru.org/images/2402863 Twilight has refined the shrinking spell to ensure the aspect of compulsion isn’t quite so intense. A few notes from Princess Celestia allows the Mane 6 to engage in periodic domination of each other as an unorthodox way to bond.

Fluttershy: [*gasp*] Applejack, you’re so tiny. Uh, why are you so far down my back?

Applejack: Yer supposed to be mah master for the day, Flutters. Twilight gave me a choice of who Ah should be subservient to after she’d gone an’ rendered herself mah slave for a day, an’ Ah reckoned ya’d be the best master Ah could get. Now, you got anything a bug-sized pony like mahself could be useful for, master?

Fluttershy: Oh, err, I don’t know. It’s rather embarrassing having you being so humble to me.

Applejack: There’s a code in place. Don’ worry none about me; Ah’m here to serve you for the day.