In verse 76, law kāna ... la-farraja constructs an unreal alternative: with another protector, distress would be relieved. Verse 77 overturns that hope through wa-lākinna mawlāya: the actual protector is himself the source of pressure.
Farraja karbī is an idiom about opening or easing constriction, while khāniqī intensifies the sensation of constriction to choking. The opposition between release and suffocation condenses the whole emotional argument into bodily images.