The
diffraction event can be described in reciprocal space by the Ewald
sphere construction (figure below). A sphere with radius 1/λ is
drawn through the origin of the reciprocal lattice. Now, for
each reciprocal lattice point that is located on the Ewald
sphere of reflection, the Bragg condition is satisfied and
diffraction arises. 
Due
to the small wavelength of electrons (e.g., λ = 1.97 pm for
300 keV electrons), the radius of the Ewald sphere (1/λ) is quite large. Furthermore, the lattice points in the reciprocal lattice of thin samples are elongated
so that the Ewald sphere intersects several of the rods (see figure). Because of that, diffraction occurs even if the
Bragg condition is not exactly satisfied, and
many reflections appear simultaneously. In fact, ED patterns correspond to
2D cuttings of the reciprocal lattice. The rod shape is due to the fact
that TEM specimens are very thin in real space, leading to an elongation
of the reflections along this direction in reciprocal space.
If the interplanar distance in direction of observation is large
(that means a small distance between ZOLZ and FOLZ in reciprocal
space), higher order Laue zones (HOLZ) can be observed as well.
A
general introduction into diffraction is given in an interactive
tutorial by Proffen and Neder.
