A bookend vortex is a local eddy caused by large variations in the wind field over a small area--for example, a SW wind at 150 km/h next to a SW wind of 30 km/h. This shear induces a vortex to form, and often the most severe wind damage (as well as occasional tornadoes) is seen here.

Whats a bookend vortex?

During the development of a bow, counter-rotating storms (or areas of storms) commonly appear at both ends of the larger-scale bowing segment, straddling the rear-inflow jet. These storms are known as bookend vortices.

Where are the strongest winds in a bow echo?

Strongest winds

Damaging straight-line winds often occur near the center of a bow echo. Damage from all severe thunderstorm winds accounts for half of all severe reports in the lower 48 states of the US, and is more common than damage from tornadoes.