Clutches*
Auto transmissions have clutch packs (a bunch of little clutch disks in a pack) that engage "gears". The only gears you will find in an auto transmission (mustang trans, not the auto-manual trans found in many cars nowdays) are the planetary and sun gears. Rare for them to actually break. Most the time you burn up a clutch pack due to it slipping. (Thats how some auto trans only lose 1 or two gears.Thats why they turn your trans pressures up. To limit the slipping and allow for more power to go thru it without slipping.)
Or you can break the shell that the band goes around. (Usually caused by rolling in reverse and going to drive too fast. Or at least what I see to usually be the cause)

The clutch packs can engage more than one gear like the 4l60e in the silverados. 9 times out of 10 you lose 3rd and 4th. Thats because the one clutch pack engages both those gear ratios.

But dont listen to me. I apparently lost my credibility on the gear tune thread

 

My auto shifts hard. It jerks me forward even when I'm just cruising. It gets annoying when your not expecting it, and my wife hates it.

 

Yup that sounds just like mine.

 

Mine does all of what you're saying stock.. just not all the time. I can only imagine what a tune thats supposed to firm up the shifting will feel like. If you don't like it, like what others have said.. ask for a softer shifting tune. Simple as that.

 

The simple fix would be to get a MT?

 

No that is definitely not the answer lol.

My car with the Bama tune does the same thing with the hard shift. I'm fine with it though. I'd much rather have the hard shifts then the lazy shifts that tend to happen at 3rd and above.