(covers information from several alternate timelines)
A subspace vortex (also called an energy portal) was an extremely fast method of propulsion utilized by the Xindi. Capable of transporting a vessel at a rate of about six light years per two minutes, the vortices were opened through the generation of a phase deflector pulse, using on-board generators, and were sustainable for hours at a time. (ENT: "The Shipment", "Stratagem", "The Council") Even if a whole fleet of Xindi ships used a subspace vortex, tracking the vessels inside the vortex could be extremely difficult for Xindi who were outside the vortex, though it was possible for an Andorian battle cruiser to carry out the task. (ENT: "Countdown", "Zero Hour")
Later that year, a pair of Xindi-Reptilian warships, after attacking the NX-class starship Enterprise NX-01, sped away using a subspace vortex. After observing the vortex, Captain Archer ordered T'Pol to find out what she could about it, as he didn't want the Xindi suddenly appearing without him being forewarned. (ENT: "Rajiin")
In 2154 of an alternate timeline, a subspace vortex was used to transport the Xindi primary weapon, accompanied by two Xindi-Reptilian warships, to Earth, moments before the weapon destroyed that planet. (ENT: "Twilight")
When Enterprise found Degra's ship in the Calindra system on December 12 2153, T'Pol speculated, in reply to Archer asking why the ship hadn't been detected before, that one of the subspace vortices may have been used by the newly arrived vessel.
Three days later, Enterprise detected another subspace vortex being used by a different Xindi craft, while it was approximately six hours away from Enterprise. By that time, Enterprise chief engineer Charles Tucker III had collected more data on the vortices, determining how they were opened and how fast they transported vessels. After viewing the data Tucker had collected, Archer fooled Xindi-Primates Degra and Thalen into believing Enterprise had been outfitted to access a subspace vortex with its main navigational deflector. The two Primates were also duped into thinking the vortex was beginning to collapse, apparently causing the ship to receive severe damage, until the craft purportedly re-entered normal space near Azati Prime. (ENT: "Stratagem")
A subspace vortex was used by a specific Xindi-Insectoid starship. However, as that ship was coming out of the vortex, one of its nacelles fractured, so the crew made an emergency crash landing on a nearby planet. While Enterprise was subsequently investigating the crashed vessel in January 2154, a subspace vortex opened off Enterprise's port bow and a Xindi-Insectoid scout ship emerged from it, prior to attacking Enterprise, which was temporarily under the command of Lieutenant Malcolm Reed. The bridge officers aboard Enterprise began to suspect the scout ship was attempting to open a vortex again when it started to charge its main deflector, so Enterprise destroyed the Insectoid craft before it could escape. (ENT: "Hatchery")
When Commander Dolim began insisting that the Xindi weapon be escorted to Earth by four Xindi ships, Degra notified him that a massive subspace vortex would be required to accommodate them and that, due to its enormity, such a vortex would be extremely unstable. Degra imagined the instability of the vortex could result in one or more of the vessels becoming lost, a possibility Dolim ordered Degra to ensure against. (ENT: "Azati Prime")
Shortly after the Xindi primary weapon was launched in 2154, a subspace vortex was used to transport it and several accompanying Xindi-Reptilian and Insectoid vessels out of a battle, as they proceeded toward Earth. (ENT: "The Council") Xindi who opposed them, including Xindi-Arboreal and Primate representatives from the Xindi Council, struggled to track the fleet while it traveled through the vortex; it was only after the vortex was closed that the fleet was able to be located by the other Xindi.
Thanks to some assistance from the Xindi-Aquatics, Enterprise made preparations to enter a subspace vortex on February 13th, 2154. The ship and seventeen Xindi vessels with it thereafter opened and journeyed through numerous vortices, with a pair of Xindi-Arboreal starships going first and Enterprise traveling inside a Xindi-Aquatic cruiser. This fleet exited the vortices in close proximity to Dolim's ship, immediately prior to battle resuming.
Later, another vortex was opened by Dolim's ship, which that vessel entered together with the Xindi weapon and a single Insectoid ship, though the latter craft was destroyed by Dolim's vessel, mid-journey. While Enterprise remained in the Delphic Expanse in an attempt to disable the Sphere network, Captain Archer boarded Degra's ship in pursuit, accompanied by Lieutenant Reed, a team of MACOs and Communications Officer Hoshi Sato. (ENT: "Countdown") Using a subspace vortex, Degra's ship, which was significantly faster than the two craft it was following, was eventually able to close the gap and detect the pair of Xindi craft while all three vessels were in the vortex. Unbeknownst to those aboard Degra's ship, the Andorian battle cruiser Kumari managed to track that craft through the vortex. The weapon and Dolim's ship exited the vortex, closely followed by Degra's craft, but not in time for Degra's ship to stop Dolim from destroying the Yosemite 3 research station.
After a battle that resulted in the destruction of the Xindi weapon and Dolim's vessel, a vortex was used by Degra's ship to rendezvous with Enterprise. Another vortex was subsequently used by the latter craft and another Xindi-Aquatic cruiser which, while traveling through the vortex, carried Enterprise in its hold. The vortex brought the two ships to Earth. (ENT: "Zero Hour")