Basically you continue to be served by your utility. Nothing changes physically. They pay your utility bill and you pay them a HIGHER price, which they use to invest in renewable energy projects.
It seems like one of those "too clever by half" ideas. If you want to support renewable energy, just buy carbon credits from an established, credible, audited, trustworthy company and don't mess with your utility billing.
What they do is not actually send you power, but just take the numbers from your electric bill and invest in an equal level of wind and solar energy elsewhere. So, essentially you pay them to offset your carbon footprint and invest in green energy infrastructure.