Semi weighted keys are referred to a spring type system which tries to replicate the feel of weighted keys. It involves a spring action key with a combination of some light-weights which offers a little bit of resistance than an unweighted keyboard. These are designed to result in a light to moderate uniform resistance and depress quickly when pressed. But they can also often feel a little sluggish as they rise back up. Irrespectively, they’re a definite improvement from keyboards that don’t have any weights at all.