Pretty much what
@UprisingJC said. It's essentially a shenanigan against people that don't know how to deal with the string. For low-high strings where the first hit leaves you unsafe, a low throw becomes an option select. On CH however, 1P is good, especially with Kasumi's strike speeds. On NH it's BS. 1P~6P isn't a thing for example lol.
That's why I have no idea how to use this move.
Her second fastest instant crouching move in her arsenal.
It doesn't track, leaves Kasumi -9 on NH and its follow-up is only one, i.e, 1pp, and lame.
I used to hope to use it to retaliate to stop my opponent from high crush, mid (punch) holds or parries and SS but unfortunately it can't.
I have to rely on her moves like 6K for the purpose. I often use it against characters like hayabusa cuz players are often forced to guess if busa's going for a high crush(33p), raw mid izuna punch hold or SS after blocking some of his safe moves such as 6PK(-5 on block). Her 1P can't deal with SS, and leaves her -9 on NH, so it's pointless to retaliate with 1p in a situation like that.
In terms of playing a stun game, I also barely use this move.
Reasons:
1.It doesn't really cause a long stun(+12 ~ +19) and the only thing guaranteed after 1p is a jab, which hits the same hit level just like her 1pp.
2.Nowadays there're still lots of low hold abusers around. I don't want to get caught by that so I don't feel like using lows for a stun game.
However, last year I used it to extend the stun when I was about to finish Fuwachin, a top Kokoro player in Japan, in Top 8 in DOA Festival just in order to surprise him, lol.