I adore my sweet yellow butter bell! I mash the softened butter down into the bell part before putting it into the bowl with the water.
By the way, that water should be just deep enough to cover the rim of the bell so that the butter is NOT exposed to the air and it should be changed every couple of days. This is how it keeps the butter from becoming rancid.
These have been used for many, many years in Europe. Gave one to my mom and daughters for Christmas two years ago.
The only problem I've had was when I let the temps in the house get too high in the summer....above 77-78....the butter became too soft for my liking.
Jan in VA