Oh, you mean she made the armor disappear and reappear in an instant? That's... quite an interesting thought.
It's obvious the joke is that she's flat chested enough there is a gap in her armor between her tits and the inner surface of the armor.
Hence there being enough space for Shirou's hand to slip in.
During patrolToday, why don't we try looking over this way?Danger!HmmObviously, this place is suspicious.Saber, let's approach with cautio...What? the heck--~~~~!! S-s-s-sorry-! I swear it wasn't on purpose...Something warm and squishy--I already know.More importantly, let's hurry and go.H-huh? You're not mad...?I'm not mad at all.Very well.