If you are stationary and push down on the pedals, most frames will move sideways at the BB. Don't know about the flexing backwards thing but it could just be in the description.
Clean and inspect the frame for fine lines, cracks, damage to lugs/joins/welds, ripples in paint/tubing indicating compression. It's probably nothing but it pays to have a look if you're worried.
If you are stationary and push down on the pedals, most frames will move sideways at the BB. Don't know about the flexing backwards thing but it could just be in the description.
Clean and inspect the frame for fine lines, cracks, damage to lugs/joins/welds, ripples in paint/tubing indicating compression. It's probably nothing but it pays to have a look if you're worried.