She also does it near the end of the Fate route in the VN, Servents only need to say the name when they want to use the full power of said move, what Saber does is just increase her normal attack while still reserving her mana and identity. It's still a risk because if you DO spot her sword when that does that you will have figured it out.
I call bullshit. Or rather, I found that part of the Fate universe general bullshit in general.
It's a silly pet peave, but I dislike anime that make the rules up as they go along for the sake of convenience, and that's one of many in the Fate universe that annoy me- right after the servant letter-class system, and of course the whole concept of the Holy Grail 'War.'
It's faux intellectualism- the sort of 'we have a great complicated system that we prove ourselves geniuses by understanding and exploiting on the narrowest of context in great daring displays of ability and intellect' that pervades this series and it's like. It's not like I expect a full rule's course in advance, but 'advantage' and 'disadvantage' in the Fate universe are less rule-driven events and more a giant game of Calvinball, making it up as they go along for plot convenience and contrivance.
A reoccuring contrivance this sort of battle anime is the idea of battlefield geniuses who make Sherlock Holmes-level deductions, analysis, plans, and counterplans based off the briefest of context and smallest of clues. It's the highlight of balancing skill and strategy, of great fights between warriors leading to clever traps hidden behind more obvious assults of skill and daring.
It's a load of bullshit. The Fate universe included loves to make grandious conclusions based off of not enough facts and data, and then pretend it was clever planning rather than luck and risk. Winners are treated as geniuses for bad decisionmaking and analysis, often because their foes are even worse, and even when it's framed as being good it's just plan annoying to me.
It's pretty animation and nicely choreographed battles, but let's not pretend that Shouting Their Attacks is determined by rules and not the preference of the author at that moment.
[/rant]