Have never seen these before - they are brilliant! It is literally like having 3 snares at a gig when only taking one. How much they lower the pitch by is quite unbelievable! And they don't make the snare sound too much quieter either. I have used the, a fair bit and they're not even marked, so very durable too.
The small tab cut out makes switching it on and off very quick and easy - I use it when performing "I love rock 'n' roll" to very quickly have that deep power snare sound temporarily for one song!