Chapter 1 What’s New for FortiOS 5.0 : Wireless : Wireless client load balancing for high-density deployments : Access point hand-off
  
Access point hand-off
Access point handoff wireless load balancing involves the following:
If the load on an access point (ap1) exceeds a threshold (for example, 30 clients) then the client with the weakest signal will be signaled by the wireless controller to drop off and join another nearby access point (ap2).
When one or more access points are overloaded (more than 30 clients) and a new client attempts to join a wireless network, the wireless controller selects the least busy access point that is closest to the new client and this access point is the one that responds to the client and the one that the client joins.