SyncScan (http://www.cs.ucsd.edu/~iramani/sync_scan.pdf), is a technique for continuously tracking nearby base stations by synchronized short listening periods at the client with periodic transmissions from each base station. In sycnscan the AP's on a particular channel send beacons at a fixed time interval, allowing adapters to only listen at discrete periods and to get infomration about all the AP's in the neighbourhood. But this coordination adds significant synchronization overhead which are difficult to implement for a large and complex network: the system relies on AP's maintaining a semi-constant 'phase' between their beacons.
Using sync_scan the overhead of scanning neighbors is staggered over short periods of time instead of only at the time of handoff. by scheduling in advance when to scan channels, a client can spend a minimum amount of time scanning channels, while still maintaining a very up to date list of available ap's, useful for mobility and triangulation techniques.
Sync_scan was originally implemented on the client side via madwifi, with a daemon manualy sending private ioctrl's to the driver to perform synchronized AP scanning. One of the key items in the system is that the cliens announces it is entering a Power Saving Mode right before switching to listen on a different channel; this "standard" feature tells the AP to queue traffic for the client until the client returns to prevent wasted data transfer. Madwifi AP's do not support this feature though, do not queue packets, and thus cannot effectively act as synch_scan ap's since they send data while the client is "away".