The sure-fire way to get these questions correct is to visualize the molecule with the lowest priority group going away from you (as described in Mike's OC video 4.7 Solution (Pt.1)). That will always work if done correctly. However, that can be difficult to visualize at times, so we have little "shortcuts" in certain cases. 1. When the LOWEST PRIORITY GROUP is on the WEDGE: you can find the R/S configuration and then simply reverse it for the correct answer. If you found the molecule was R with H on the wedge, then you would switch the configuration to S and that would be the correct answer. However, when the LOWEST PRIORITY GROUP is in the plane and NOT ON A WEDGE, we cannot just switch the configuration to get the correct answer. What do we do in that case? We can either visualize the molecule in our minds with the lowest priority group going away from us (as done in the video) OR... you can switch the lowest priority group with the group on the dash, redraw the molecule after making this switch, find R/S configuration of this new molecule, and then reverse it.
For example, with answer choice B in video 4.7 Solution Pt.1, we could swap the H with the NH3+. Try drawing this new molecule (NH3+ and CO2- should be in the plane of the board, CH3 on a wedge, and H on the dash). This new molecule would have an R configuration. But because we swapped the H in place of the NH3+, we need to reverse the configuration, leaving us with S as the correct configuration.