The word represents the feeling you have when a person whom you expect to perform well does, in fact, perform well, and you feel a sense of awe all over again at their amazing feat.
To learners of the language, sasuga is a little frustrating in that it's one of those ridiculous ateji words whose reading and kanji characters don't match. But there's a cute little story about why sasuga is written this way today:
A long time ago in China, there was a guy named Sosen.
Sosen was a genius, and knew he was way smarter and better and more humble than anyone else around him. So much, in fact, that he found it hard to get along with others. Above all else, he hated it when someone else tried to point out when he was wrong. (As if he'd ever make a mistake in the first place.) So anyway, one day, when Sosen was feeling really fed up, he decided he'd had enough. I'm gonna go live in the mountains, he told himself. I'm gonna go be one with nature so I can finally stop dealing with all these idiots and have some goddamn peace and quiet.
Despite hating pretty much everyone else, Sosen did have one close friend. His name was Ousai. And Sosen liked Ousai mostly because Ousai actually, sincerely believed that Sosen was an absolute genius. So before Sosen left to begin his hermitage in the mountains, just so people wouldn't think he'd, like, died and get depressed over the loss of such a beautiful mind, Sosen decided to tell his friend Ousai where he was going.
"Yeah, so I'm gonna go into the mountains. Just kinda get away from this crappy world we live in so I can really focus, you know?" he said.
And Ousai was like, "Whoa, that's amazing." But then he thought about it a little more and said, "Wait. The mountains? Man, how are you gonna live out there? Where are you gonna stay? What are you gonna eat?!"
Sosen had this all thought out. Obviously he'd be able to hunt for food, and he said so. And as for water and a place to sleep, he'd just use rocks for pillows and drink from streams in the mountains. And he told Ousai this, too. But when he said it, it came out wrong:
"I mean, duh. I'll just lay my head in streams and fill my mouth with rocks."
And Ousai was like, "...What."
Sosen realized what he'd done. But he wasn't about to get called out now, on the very cusp of his new, enlightened lifestyle. So he came up with a quick save:
"Dude, Ousai. I mean I'll be able to wash my ears in the stream water, and I can use rocks to keep my teeth sharp for all that tough meat I'll be eating out there. Keep up with me, man."
To which all a nigh-speechless Ousai could respond was: