I use a POE switch, because I don't external power ...
Same thing. You can disconnect the ethernet cable to power off the WAP or you can use the command in the interface-level "no power inline" or "power inline never" to disable power to the port.
1. Connect the WAP into, say, port 1 of the Cisco switch.
2. Connect the TFTP server to say, port 2 on the Cisco switch.
3. Make sure they are on the same VLAN, VLAN 1, for example.
4. In the switch, configure this:
power inline never
5. Hold down the WAP's "mode" button
6. Back to the switch, enter the command: no power inline never
7. Keep holding the mode button until it'll say something like "waiting for the mode button to be released" or something.