[quote=@Turboshitter]
<Snipped quote by Unoedipal>

Lancelot [i]chose[/i] to keep it on though, didn't he? And Illya kept it on constantly to punish Berserker (even though doing so hurt her), so it must have a toggle switch.
[/quote]

Yeah, he did. That's why it isn't a solely Master privilege. 

Illya's a cup and even if she wasn't, that whole deal got retconned in RN. I'll post quotes at night when I'm done with work if you want. 

But yeah, only instance we've seen of it being a toggle is Fran, whose CMIV profile says its toggleable.