Second what Andy said above.
Add to that shifting to low but not back, with a slack cable is a classic sign that the cable is binding someplace.
In fact, the bike is speaking to you, and telling you that the pinch is someplace between the slack area and the derailleur, with the frame/derailleur loop being the likely suspect.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.