So (using easy math), if you made 500,000 a year, and your nominal tax rate was 35%, but by utilizing a series of shelters you were able to lower the tax money you actually sent in to 100,000, then your effective tax rate is 20%. The Ryan plan is to simply drop your tax rate to 20%, while also removing all of the shelters you use to lower your tax bill. What You Pay stays the same, it's simply that now you aren't wasting resources trying to move everything into one shelter or another.
mind you if we end up increasing the burden of the middle class relative to the upper income earners, i am also fine with that. we currently have the most lopsided, 'progressive' tax code in the industrialized world, after all. It just doesn't happen to be part of the current from the House of Representatives.