Remember that insane video of Amir Khan chugging a protein shake and then juggling the bottle in midair with punches???
Yeah ... dude says it was 100 percent real -- telling TMZ Sports it took AN HOUR of practicing before he got it right!!!
The trick is wild ... Khan tosses a plastic bottle in the air, hits it just right so it stays spinning ... and then he keeps it in the air with punch after punch until he's bored with it.
Khan tells us the trick ain't easy to do ... saying, "I was standing there for about an hour until I got it right!!"
In fact, Amir says it's SO difficult, he's only been able to do it twice since!!!
As for his upcoming mega-fight with Terence Crawford on Saturday night ... Khan says he's feeling extra motivated to get the win because he's such a huge underdog.
And ... if he does pull off the upset -- Amir says he might just do the bottle trick as an in-ring celebration!!!