Lots of riders seem to be confused by the dressage term "long and low". Some riders even think long and low is different from the stretchy circle in the dressage tests.
Long and low is the SAME as what's being asked for stretchy circles. The stretchy circles were added to the dressage tests to check that the horse's connection was correct and that the rider wasn't fudging things by fiddling with the reins.
To get correct long and low, your horse must be connected over his back. So give the connecting aids and then allow the horse to chew the reins forward, down, and out.
If you sponge or play on the reins, you're just flexing the horse's jaw. That has NOTHING to do with correct connection.
To do correct long and low, close both legs to send your horse forward through the closed outside rein while keeping him flexed to the inside. The heart of the connecting aids is closing your legs to send the hind legs forward as if you're asking for a lengthening and then recycling that energy back to the hind legs with a closed outside fist. That's the part that gives you longitudinal bend over your horse's back.
Once you've given those aids, open your fingers and allow your horse to stretch long and low. You're the one who decides how much rein to feed out.
If you're successful using your connecting aids, stretching long and low is the natural progression of the longitudinal bend you've just created over your dressage horse's back.
A Happy Horse