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If I were to attack a squad with a group of guants, could I then have my tyrant fly over the combat and assault from behind?
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Yes. The answer can be found in the 4thEd rules, page 55, "Jump Infantry", left column, "Movement Phase", lines five and six "...and jump pack equipped..."
How does this relate to Tyranids? As we all remember, 4thEd defines Winged Gaunts, Warriors and Tyrands as "move as JumpInf"... Thus they get to move over models.
However, as we all remember, the Winged rule still restricts Tyranids from entering terrain, and as a Codex special rule overrules the generic JumpInf rules from the main rulebook in this regard.
"The meat things seek to destroy ourselves. They hunt us as we hunt them but they are weak and uncertain. The inferior flesh will be entirely destroyed, all biomatter consumed. Ourselves will fight to the last, all weak flesh must be devoured."