Whether you are hitting or looping, you should normally line up your shoulders when you backswing on the forehand so that the shoulders roughly aim in the direction you are hitting towards. This maximizes your hitting zone and allows you to stroke naturally through the ball. If you don't rotate the shoulders back enough, you'll have an abbreviated stroke, costing you control and power. (The reverse is less often a problem, but if you do rotate the shoulders back too much, then the stroke becomes too long and cumbersome to control, as well as taking too long in a fast rally.)
So a top priority in developing the forehand is proper shoulder rotation. But once the shot is developed and the shoulder rotation natural, you can use this very shoulder rotation to deceive an opponent.
Imagine lining up to hit or loop a forehand crosscourt from your forehand side. You line the shoulders up during the backswing, and are about to start the forward swing. Your opponent sees your shoulder rotation, sees that they are lined up to hit crosscourt, and instinctively moves to react to a crosscourt shot since most players hit the ball wherever their shoulders line up to hit. Then, at the last second before starting your forward swing, you rotate the shoulders back a bit more, line them up for a down-the-line shot, and then hit down the line. Instant free point.
The key is that instant of "hesitation" where you have stopped your backswing with the shoulders lined up crosscourt, where you let the opponent react, and then the final extra bit of shoulder rotation before going down the line. The timing is surprisingly easy as long as you focus on lining up the shoulders properly for whatever direction you are going.
The shot can also be done with the forehand from the backhand side, where you aim down the line and at the last second go crosscourt. In both cases you are faking to the left (for a righty), but going to the right.
The alternate version is to line up your shoulders to go down the line from the forehand side, and simply rotate the shoulders forward more during the forward swing so that you hit crosscourt. Or from the backhand side, line up the shoulders to go crosscourt and go down the line. In these two cases, you are faking to the right (for a righty), but hitting to the left.
When doing these deceptive shots, note that some opponents automatically cover the wide crosscourt angle no matter how you line up your shoulders. Against this type of player you should mostly fake crosscourt before going down the line. Some might be so ingrained to cover the crosscourt angle that no deception is needed, just go down the line. But when/if they adjust to that, then you can fake the down the line and go crosscourt.
One consequence of the tendency for some opponents to cover the wide crosscourt angle is that it is sometimes less effective to fake down the line and then go crosscourt, since the opponent might be ready for that. This is especially true when doing a forehand from the backhand side, where even if you fake it down the line many opponents still guard the crosscourt angle. On the other hand, if you rotate your shoulders way back to fake down the line from the forehand side, most opponents tend to react to this down the line fake, leaving the crosscourt angle open. This is because few players over-rotate the shoulders (which is essentially what you are doing here), and so opponents are more likely to fall for this.
There are other ways of misdirecting an with your forehand. For example, you can learn to hit inside-out, where your shoulders aim left (for a righty) but your arm and wrist twist back at the last second and you go to the right, often with sidespin. However, that takes tremendous timing, and while many top players master the shot (especially when looping, where they have extra topspin to pull the shot down if it isn't timed perfectly), the simple last-second shoulder rotation allows you to get almost the same misdirection without developing the difficult timing of that inside-out shot.