Thanks to Hey & Chiu for posting photos & a sound recording of this intriguing bird.
It was certainly a very small & slim Cuckooshrike, with blackish upperparts & a narrowish pale rump band which contrasted very strongly when the bird was in flight, and a strange call or song. This was a simple 3-note phrase frequently & without variation for 1-2 hours (as on on Hey's sound recording).
I want to comment especially on voice. I don't recall hearing Black-winged Cuckooshrike vocalising in Hong Kong either on passage or in winter, but I have heard & recorded on MD one singing in late April at Ba Bao Shan, northern Guangdong, presumably on territory. I don't know how to transfer an MD recording to computer, so here's a verbal description. The main phrase is a fairly loud, rather high-pitched and clearly-separated 2-note whistle "gee-heu", uttered once or repeated 3-4 times. When repeated in a series, each phrase descends in pitch. Also, there is sometimes an introductory phrase - a faster & slightly quieter 3-note "Whi-hi-hi" which preceeds the main phrase, so the whole song is "Whi-hi-hi ... gee-heu ... gee-heu ... gee-heu... gee-heu", which is very different from the Po Toi bird.
Robson's (2009) description of voice of Black-winged Cuckooshrike, whilst matching the Ba Bao Shan bird, does not sound anything like the Po Toi bird: "Series of 3-4 clear, well-spaced, high-pitched whistles: "wii-wii-jeeu-jeeu", "wi'i-wii-wii-jeu", and "witi-jeeu-jeeu-jeeu" etc; slower & more measured than Indochinese (Cuckooshrike)".
So, size, upperpart appearance & voice of the Po Toi bird all seem wrong for Black-winged Cuchooshrike!
