4p4p1 in PPS

In Pass-pass-self (3p,3p,3), there is one self and so one can turn the sequence into Pass-Pass-Self into Pass-flip-doublepass (3p,2,4p).

You are throwing the pass one beat earlier (instead of the selv), but you are also throwing the pass so much higher, that it arrives one beat later. The self is now thrown one beat later, but thrown such that its flight time is one beat shorter than usual - which turns it into a Flip. As a flip arrives at the same hand without intermittent throws, you can just skip it and do a hold (i.e. do nothing on that beat).

Regular Pass-Pass-Self:

A 1|2 P P S | P P S
B 1|2 P P S | P P S

With one early double (Juggler B has the changed pattern, A keeps doing PPS):

A 1|2 P P  S | P P S
B 1|2 P P dP | F P S

Now there is the sequence "Flip, Pass" as part of that pattern and you can do the same thing again: you throw the Pass one beat earlier, but higher so that it arrives at the same time as before. The "self" is thrown yet one beat later and again with one beat shorter flight time and so turns into a zip:

A 1|2 P  P  S |  P  P  S
B 1|2 P  P dP | dP  Z  S

"Demoting" the same pass, which is now a zip (siteswap 1) again would make it a siteswap 0 - an empty hand. And we now have not a pass lined up, but a self, which would turn into a heff.

A 1|2 P  P  S |  P  P  S
B 1|2 P  P dP | dP  H  0

Generally patterns with an empty hand do not feel nice, though. A solution to this would be if B picks up another club and throws:

A 1|2   P  P   S
B 1|2  dP  H  dP

Mind, that B now has an 8-club passing pattern while A has a 7-club passing pattern, as is easily seen: if A wants to throw the same sequence, they have to pick up another club.