Some performances are so complete and flawless that there's genuinely nothing left to critique. Ate and left no crumbs captures exactly that level of praise, using a vivid food metaphor for total, undeniable success.
What this phrase actually praises
This phrase praises a performance, outfit, or execution so flawless and complete that there's nothing left to critique — the metaphor of eating everything with nothing left behind represents total, undeniable success in whatever's being described.
Where the food metaphor comes from
This phrase builds on "ate" as existing slang for doing something exceptionally well, extending it with "left no crumbs" to emphasize that the execution was so thorough and complete it left absolutely no room for any valid criticism.
How this phrase compares to just saying ate
"Ate" alone already conveys genuine praise for doing something well, but adding "and left no crumbs" emphasizes an extra degree of totality and flawlessness, signaling that the execution wasn't just good but genuinely complete and beyond critique. 
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Escalating praise through the full phrase
Using the complete phrase rather than just the shorter version signals a stronger, more emphatic level of praise, reserved for performances that feel genuinely flawless rather than merely good.
Flexible across many kinds of achievement
This phrase works for praising anything from a performance or an outfit to a sharp argument or comeback, as long as the execution genuinely felt complete and left no valid criticism behind — its flexibility is part of its appeal.
Similar flawless-execution praise slang
This phrase sits alongside other emphatic praise slang, similar in spirit to how what does understood the assignment mean praises a different but related kind of genuinely well-executed performance.
Conclusion
Ate and left no crumbs gives genuinely flawless execution the emphatic, vivid praise it deserves. For more slang and meme phrases explained clearly, browse SoundGIFs' full News section.