「缺」 (Baxter-Sagart OC: /*Nə-[k]ʷʰˤet/, to damage/break vessels [pots, dishes, urns, etc.] > flawed, deficient, lacking) is comprised of semantic 「缶」 (clay jar) and phonetic 「夬」 (/*[k]ʷˤret-s/). That is, the phonetic component 「夬」 originally matched the pronunciation of 「缺」 much better, and this match occasionally survives in other characters like 「決」.
「夬」 itself now sounds much more like 「快」.
I don't understand what "夬叏" is
Confusing annotation by Hanziyuan #1: Randomly listing variant characters with absolutely no explanation.
「叏」 is the component-transcribed older form of 「夬」.
「叏」 originally depicted a hand 「又」 sporting a thumb ring (the archery accessory). The thumb was later emphasised as a vertical line and then detached to be part of the ring.
Later on, the hand merged back with the emphasised thumb, but the middle finger of the hand became less emphasised and obscured, forming 「夬」.
what does "phonetic-foot-hand" mean
Confusing annotation by Hanziyuan #2: Attempting to list a meaning alongside a component whose purpose is phonetic, and also getting the meaning wrong through a misunderstanding of the decomposition.
Foot-hand is what Hanziyuan thinks the meaning of 「叏」 is, and phonetic indicates that 「叏」 is the phonetic component of 「缺」. The interpretation of 「叏」 as foot-hand comes from an incorrect decomposition of 「叏」 as 「夊」 (foot) and 「又」 (hand).