Bound Spike takes the Spike tip and mounts it on a retracting shaft. The tip itself is tiny and sharp, protruding only slightly from a broader collar, so most of the time it sits still like Needle or Spike would. Tilt it, or launch it on an angle, and that collar catches the floor, friction spikes, and the bey lurches into sudden movement. The spring gimmick is meant to compress on landings and absorb hits from above, cushioning the impact.
Whether that cushioning does anything is genuinely unclear. Documentation confirms the shaft does retract on landing but describes its measurable effect on defense as hard to detect. What is clear is the cost: the spring holds the bit extended for the duration of a battle, which raises the whole combination and makes it noticeably easier to burst and destabilize.
The sudden accelerations can produce counter-attacks, but they can equally produce self-knockouts, and sources are blunt that Bound Spike is not a competitive recommendation. Anyone wanting genuine defense from a similar profile should be running Needle or Wedge instead.