When you stop at a red light in your car and your FM channel, suddenly fades then resumes. This is known after Stephen Rice as Rician Fading loss by the same RF amplitude being inverted from multipath with phase cancelling for some of the wavelengths of all FM RF signals.
In Wifi, it can be demonstrated MP with software tools that convert the WiFi Received Signal Strength Indicator (RSSI) Voltage into dBm instead of 1 to 3 bars. It doesn’t have to be the entire spectrum of ODFM wavelengths that cancel , but enough to distort some of the packet while the rest of the packet could be full,strength giving you no indication at all of a poor signal with full bars. -70dBm is marginal and -80dBm is easily lost with Ricean Fading.
Often all it takes is at this level is a 1 degree orientation or a 1 or 2mm change in position change in the Wifi dongle or laptop and your signal is full speed again.
welcome to Rician Fading loss, but all the best methods of modulation are used to thwart this. The best mobiles and laptops had orthogonal diversity antennae (two) to choose from, in case of this loss of signal.
Although I have used Netstumbler to receive Wifi RSSI levels so I could reach a far away signal by bouncing off backyard trees when I had moved in before service installed at a time when open Wifi was default on Routers.
But the trickiest example of Rician Fading was an expensive Microwave tower link that was installed in summer and failed in winter. Ground Ice cause a surface reflection with the exact wavelength to cancel the received signal. Often it is the reverse with water instead of snow with ground reflections. But if you ever want to see a Garmin GPS get lost in skyscrapers and move your position 2 blocks over due to skyscrapers in Toronto, welcome to multipath Doppler position error.