JHCCoffee
It is not designed by the manufacturer for this purpose but MacGyvered by me .
It works exactly the same as a blind filter backflush but with an extra 100mls of liquid.
It is basically a piston in a closed cylinder.
When you attach it to your group and activate your pump the piston is pushed by the water and the air on the other end is compressed.
When you release your E61 lever it opens the group and the compressed air pushes the water all the way through the group to flush.
(Think about putting your finger over the end of a bike pump and then letting go of the handle.)
The Weber device works the same but with a spring to power the flush rather than pneumatics.
I am not familiar with the Lelit E and its recommended flushing routine with the solenoid group so, while I can’t see a problem, I would not advise it without checking with Lelit or another expert.
