When a hitter has worked a full count with the bases loaded, the pitcher’s options have narrowed dramatically. This is when a batter needs to understand the situation.
Colin Moran recently took a 95 mile per hour fastball out deep to right field for his first Pirate’s home run. The icing on the cake was that it was also a grand slam. This is a great example of situational hitting.
With the bases loaded and a full count, the hitter knows they are in a fastball count. When Big Mike was playing for Ted Williams and the Washington Senators and you were late on a fastball in a fastball count, he would come unglued (to put it lightly).
Having solid swing mechanics is great, but you have to be a smart hitter and give yourself a chance to succeed. If you can anticipate a pitch, you can relax and let your mechanics take over so you destroy it.