This is a radar image from Massachusettes yesterday. The bright purple color you see at the end of the hook echo looks like a "debris ball". A debris ball is a round area of high reflectivity on Doppler radar at the end of a hook echo. The radar is actually "seeing" large debris getting picked up by the tornado! This can include trees, insulation, parts of houses, etc.
We saw a simliar debris ball during the Joplin, MO tornado. Joplin is the little spec northwest of the debris ball.