I’m using Japanese Nigari, which is a purified sea salt extract of Magnesium Chloride and other trace minerals. Magnesium Chloride ionizes 100% in water so I think it’s quite bio-available.
As for what it does, I read it contributes to being more relaxed and improves sleep. It also plays an important role in bone density and avoiding calcium deposits in the wrong places, like articulations. We need magnesium intake of about 300-400 mg a day. Most people don’t get it.