A massive, unstable burst of energy with the appearance of a fountain of water, preceding safe transport through the Stargate. Following the lock of the final chevron, the vortex emerges and literally leaps beyond the ring, devouring any matter in its path, before it dissipates back into the "pool."
The exact purpose of the energy vortex is unknown, though it is logical to assume it is yet another safety feature that generates space for an occupant to arrive. The vortex prevents the item arriving from smashing into any matter on the other side, being reabsorbed back into the gate, which would destroy the traveler.
The iris and Stargate shield, leaving only microns of space from the mouth of the event horizon, prevent an energy vortex from forming.
Prisoners - Captain Carter explains the scientific instabilities behind the energy vortex just before several prisoners of the Taldor relinquish their lives to it, allowing their bodies to be absorbed into the wave in the hopes it will take them to another planet.
Secrets - Disguised as a Horus Guard, Teal'c pushes one of Heru'ur's warriors into the forming vortex.
A Hundred Days - After using a particle accelerator to create a pocket of gas in the naturally formed "iris" of Edora's Stargate, the Earth gate again connects with the planet, allowing the Edoran gate to create an energy vortex which opens a much larger cavern. This allows Teal'c the means to journey through and begin to dig his way to the surface of Edora.
The Light - Barber, possessed by the euphoria of the light emitter, plunges himself into the opening energy vortex.
Origin - Teal'c and Cameron Mitchell toss an Ancient communication terminal into the vortex, severing Daniel and Vala's connection to the Ori Galaxy.
The Pegasus Project - A trans-galactic dial from the Pegasus Galaxy destroys an Ori warship using the unstable vortex of the Supergate.
Stargate: Continuum - When the Stargate is dialed on board the sailing vessel Achilles, the vortex punches a hole in the side of the ship.
Cloverdale - Sgt. Greer repeatedly dials the gate to keep predatory alien creatures at bay, even destroying one of them with the vortex when it tries to attack.