This is my own current understanding of what is going on in saifa, based on my experience of it solo and the bunkai I have learned from it. I haven't had a chance to talk about any of this with my instructor yet, so in a later post, I may change my views (or maybe not - I can be quite stubborn at times).
Saifa can be broken down into 6 (7 with the mawashi uke at the end) technique sequences.
The first is the initial angled shift forwards, rotating the elbow out and stepping back into shikodachi with a backfist strike.
Sequence #2 is the up-down open hands with a kick
Sequence #3 is the double leopard-fist strike and circular fist-to-open hand motion
Sequence #4 is the turn, hammerfist and short rip.
Sequence #5 is the deflection and punch
Sequence #6 is the kosa-dachi, pivot and bringing the right arm in front of the body.
Technically, the mawashi-uke at the end would be sequence #7, but I haven't learned a saifa-specific bunkai for it, and it could fit in with the finish of Sequence #6 if you are imaginative.
So, I see 6 main technique sequences. What do they have in common?
As I have been taught them, sequences 1, 2 and 4 are all defences against some sort of grab. Sequence #1 is against a same-side wrist grab, #2 is against a double hand grab (of both wrists) and #4 is against a shoulder grab from behind. As I have been taught them, sequences 3 and 5 are against punches, but could also work as effectively against lapel/throat grabs. 6 is taught as against a punch/push to the chest, but I have a theory (and only a theory, but based on similar techniques I learned doing aikido, and referencing one of the two-person drills from Choki Motobu - #12 to be precise) that it could be applied to an early attempt at a rear bear-hug.
Sequences 1,2,3 (in application, but not in the kata), 4, 5 (in application only) and 6 are all moving around/off the line of attack. In the kata, #3 and #5 move the line of the attack off-line, but work best in application if they are accompanied with a slight angled evasion.
Why train against grabs? Why would someone grab us? Generally, the answer is, so they can either pull/push us off balance, or more likely, pull us into a strike of some kind. So I see saifa as a kata where the core principles are:
Against a grab -
move off-line to avoid the follow-up
immediately off-balance the opponent using that initial movement.