When you jack up the car, the strut is coming down. When you take the jack off, the strut tries to go up, but the cable prevents it. Thus it won't let the strut compress.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne