These commands will usually boil down to a single
ioctl() system call that will clear (1st command) or set (2nd command) the IF_UP flag in the network driver.
What happens after that is up to the driver. It may or may not power cycle the network interface card. It probably won't "stop" (unload?) and "start" (load?) itself (but something else in the system might, in reaction to what the driver does e.g. hotplug code).
The driver is likely to remove the route to the interface that it put in when the interface was originally brought up, and some sort of event is likely to be created depending on the kernel (e.g. hotplug or udev etc).
On Debian it's probably better to use
ifdown wlan0 and
ifup wlan0 instead.