Explanation of character decomposition for 缺
「缺」 (Baxter-Sagart OC: /*Nə-[k]ʷʰˤet/, to damage/break vessels [pots, dishes, urns, etc.] &gt; 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 &quot;夬叏&quot; is

Confusing annotation by Hanziyuan #1: Randomly listing variant characters with absolutely no explanation.
「叏」 is the component-transcribed older form of 「夬」.
商甲前4.1.2合集21367戰國・楚簡2.260包山竹簡戰國・楚簡2.2望山竹簡秦簡18.158睡虎地秦簡西漢隸老子・甲43馬王堆帛書隸定　
「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.
東漢隸易經熹平石經楷　
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 &quot;phonetic-foot-hand&quot; 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).

References:

季旭昇《說文新證》
小學堂字形演變
郭沫若《甲骨文合集》