The best way to ensure that your clients (laptops) pick up the strongest access point (AP) is to perform a site survey, and configure the TX power and choose a channel that provides the correct coverage. You need to keep in mind that it is the client that chooses to roam, and that on some client utilities, you can have some adjustment. There is no adjustment on the AP that ensures that a client uses the AP with the strongest signal.
Again it is the client that decides when to roam and which AP to connect to based on what it perceives the signal to be. The Wireless domain services (WDS)and Fast secure roaming utilize cached credential to aid with fast roams, but do not tell a client when to roam or which AP to connect to. These adjustments can sometimes be performed on the client utility.
One should understand that the re-association occurs only when the signal from the first access point becomes null (signal=0). If there is not an active data transfer, then yes the signal strenght must be pretty weak. If there is an active link (high data rate), then the roaming will be occur based on the RF level retries and AP signal strength.
What happens in the background that the client will move to another AP based on information the client receives from the AP beacons. The AP embeds in its beacon some Cisco proprietary information concerning signal strength, number of users, % of time spent transmitting and bit error rates. The client analyzes the information from all AP beacons it receives and then associates with the one that is the "best".
The following information is included in the beacon:
Backbone Connection ( Is the AP on the backbone ? ) RSSI ( Signal Strength ) % of transmitter loading ( Percentage of time the transmitter is actually in use ) Number of users associated to the AP, using this data, the client will roam.