From Lexicanum

A Flip Belt (Eldar: geirgilath, "belt of speed") is an anti-gravity belt used by the Eldar Harlequins.

Flip Belts are suspensor, or anti-gravity belts which allows wearers to pivot effortlessly at hip level, as well as increasing the Harlequin's already considerable agility. A short range negative gravity field is generated, allowing the Harlequin to use their already prodigious acrobatic talent to greater effect, bounding across battlefields and performing killing blows that are too fast and complicated to be blocked by enemies in combat.

Sources

• White Dwarf 105 (1988), Harlequins, by Graeme Davis - reprinted in Warhammer 40,000: Compendium, p. 184
• Codex: Eldar (4th Edition)
• Harlequin Army List (PDF) - no longer accessible