Overshooting thunderstorm tops occur when a strong thunderstorm updraft has enough momentum to caryy rapidly rising parcels of air past their equilibrium level at the tropopause:
Visual examples:
Diagram:
On satellite imagery overshooting tops cast shadows nearby if the sun is low in the sky. Therefore they are best seen early in the morning or late in the afternoon. The overshooting top takes up only a small portion of the entire area covered by the cirrus anvil.
Loop of overshooting cloud tops during the Daytona hailstorm of May 4, 2005.
Other examples:
View over Brazil from the Space Shuttle in 1984.
Overshooting tops associated with tornadoes.
Overshooting tops in tornado outbreak satellite animation.
Another example animation.