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CHKD HEELS LV BKSTR

Checked Heels ? backstretch.

Not quite sure what this means either.

You're on the right track.
Not sure about LV, but I do know there were some new folks hired for charting not long ago and ever since some of the charts have been something between entertaining and downright puzzling.

Believe that might be short for leaving backstretch

I believe you are correct Lt1.
Used to be they would have noted it "chkd FT", if at all.
I can't recall ever having seen "LV" on a chart before.
Nice to learn something new every day!

What is CHKD Heels referring to?

Horse behind overruns the horse in front of him. Dangerous thing in that it can, and does, cause spills injurious to both horses and riders. It often results in injuries of varying severity to the horse in front by way of the toe grabs slicing heel bulbs, or even tendon/ligaments along lower rear of the trailing legs.
A horse can injure himself by overstep in his stride when a hind catches his own front.
Bandages can serve as a somewhat preventative measure to reduce tissue damage from such an incident, but they do nothing to prevent the trip/spills.