In more detail, an IRAT handover works as follows:
On Call initiation, the RNC sends predefined thresholds to the UE for Event 2d RSCP and Ec/No. Event 2d tells the UE when to request entry into compressed mode so that it can simultaneously measure GSM neighbour signals.
When the UE received signals fall below the Event 2d thresholds, the UE sends an Event 2d to the RNC which responds with a compressed mode entry sequence, followed by sending the UE a GSM composite neighbour list – which contains the thresholds for another event – event 3a – which is the trigger level at which the UE will request an IRAT handover.
The UE then starts scanning for the GSM BCCH-BSIC combinations contained in the neighbour list and GSM neighbours which fall above the broadcasted GSM signal minimum level will be reported to the RNC as soon as the UMTS signal falls below the Event 3a threshold conditions.
Once the RNC receives an Event 3a, it sets up the handover call via the MSCs to the associated GSM BSC, requests resources on the GSM base, and when all of that is confirmed, the RNC issues a handover command to the UE, instructing it which GSM base station to hand over to.
The UE then relocates and sets up it’s call on GSM and the connection then continues on the new RAN. If the handover fails, the UE relocates back to UMTS and the RNC will prepare to try again – either to the same GSM signal or to another one of the UE sent multiple candidates in the Event 3a.
Thats a broad outline of the steps involved in an IRAT handover – hope that helps. For more info refer to the RRC management 3GPP standard (sorry cant remember which one it is).