Concept of Flexible Abis is in fact an Abis pool for one TG. One TG corresponds to one E1 and can be used by multiple synchronized cells. Purpose is to make a pool for E-PDCHs and G-PDCHs. Any cell can used a 16K or 64K TS from the pool whenever needed i.e. saving Abis by trunking.
Ericsson claims that it saves almost 35% Abis resources with less than 15% loss in EDGE performance where usage of EDGE is 15-20%.