Garage frame lift job

Crushed shackle

 

Don't know where my words went. I'm doing rear leaf shackles with the frame supported at three points. When I cut the shackles off, will they jump or leap out at me or my hands (or face?) It's not like struts, right? It definitely shouldn't thwack the bed, right? It should relax downwards, right?
Common sense tells met the axle will pull it down and it might relax by a few inches, but you can't see potential energy (well I can't anyway and common sense isn't my strong suit). So any advice is welcome.

 

It's going to drop, I'd put the floor jack under the differential, don't lift it, just jack the jack up until it touches.

 

+1 ^^^^