If the client roams then there will be an exchange of the clients details between the 2 AP's using Inter AP protocol IAPP and any packets sent to the old AP will be forwarded to the new AP via the ethernet side
If the client simply disassociates then associated to the new AP then it has not roamed. So this information is not exchanged This would happen if your 2 cells dont overlap.
If you look in the logs you can see a roam message when a client roams.
If you are using Non Cisco clients then they may not be able to roam. The standards on this are very close to be finalised now so in the future this will be the case for all clients