Arguments for No Quickening:
* The Quickening is linked to death: The Quickening is the energy released when an immortal dies, allowing another immortal to absorb it and gain strength. If the immortal is beheaded *before* experiencing true death, it's debatable whether they would release the Quickening at all.
* Beheading is a "clean" death: In the Highlander universe, beheading is often presented as a "clean" and instantaneous death, bypassing the usual pain and suffering associated with mortal death. This suggests the immortal might not experience the necessary "death process" to trigger the Quickening.
Arguments for a Quickening:
* Immortality doesn't mean invulnerability: Even though immortals are practically invincible, they can still be killed. Beheading, while quick, is still a form of killing, even if it bypasses the typical suffering.
* The Quickening is a supernatural event: The Quickening is an unexplained supernatural phenomenon, and it's possible that a beheading still triggers it, even if the immortal doesn't suffer a prolonged death.
Conclusion:
Ultimately, the answer depends on how you interpret the rules of the Highlander universe. There's no clear answer provided in the show or movies, and the question remains open to interpretation.
Here are some potential scenarios to consider:
* The Quickening is released but is weak: The beheading might trigger a faint or incomplete Quickening, not enough to be useful to another immortal.
* The Quickening is released but unusable: The Quickening might be released, but it's somehow "contaminated" by the beheading and cannot be absorbed.
This lack of clarity is part of the mystique of the Highlander universe, encouraging fans to debate and theorize about the nature of immortality and the Quickening.