The change was made to make self voting less attractive. If whales vote on their own post they now receive only 75% of the value of the vote. 25% of the value of the vote goes to the curators or to the reward pool if the voter votes within the 15-minute timeframe.

what happens in the second 15 minute period now?

There is no second 15 minute period. Either one votes within the first 15 minutes or after 15 minutes. This was 30 minutes before HF20, but there is now no 30-minute timeframe anymore.