not to throw the spanner into the works,
but....in our Hung-Ga, the opening sequence of FHSYK is all about waist!
The body is very serpentine when shifting (opening movement is in YJKYM, press, mai yun jiew gang, chum kiu, thengrab and pull to waist while shifting-some shift from YJKYM to Gung-Bo, but we pretty much keep YJKYM and allow the waist to do this.) This is similar to single hand tui-sao in Tai Gik Kuen, if you catch my meaning, and can be used when crowded/cornered, with nowhere to escape, you coil and shift.
The next move si twist the opposite way and palm strike, then face center press palm, etc.
The second section also has a strong emphasis on waist coiling with the left and right soft bridge.
Just my .02