Appears that the new system does not pre-allocate seats, rather it notionally allocates seats, the seat is only registered as taken when the passenger actually checks in (hence the increased availability to pick seats)
eg. on a full flight with 40 rows
20% WP, 30% SG, 20% PS, 30% nada.
First WP to check-in has choice of the whole plane.
SGs can pick from 8 rows back
PSs can pick from 20 rows back
etc.
Means much more benefit from OLCI at 24hr mark