Second Magic

The Second Magic (第二魔法), known variously as Zelretch and Kaleidoscope (though it's unknown if these are its true names) is one of five remaining Magics in modern Nasuverse.

History
Kishua Zelretch Schweinorg had already achieved it several centuries ago when he used it to kill Brunestud of the Crimson Moon, though exact details are not available. It is said however that even Archer, the only known Servant from the future, would be unable to comprehend it.

Effects
The little that is known about "Kaleidoscope" is that it encompasses the Operation of Parallel Worlds. Zelretch himself uses it frequently to move through parallel worlds, though due to his age he cannot use it as extensively as in his youth. Two of his inventions, the Kaleidostick and his trunk, also use the Second Magic to some degree. After the Fifth Holy Grail War, Tohsaka Rin attempted to create a pendant, a "miniature of a miniature" of the Jewel Sword that would allow her to observe the 'waves' from parallel universes, though this failed spectacularly.

Dimensional Refraction Phenomenon
The Dimensional Refraction Phenomenon (多重次元屈折現象) is one fragment of the Second Magic, essentially the ability to create openings into parallel universes. One can then interact with the 'other side' through this gap.

The Jewel Sword, and Tsubame Gaeshi, Assassin's sword technique, are both capable of causing this phenomenon.