Difference in Ma Long's serve direction

i had watched malong's long forehand serve down-the-line, long forehand serve cross-court and they look quiet similar. the problem is i can't figure out what makes the difference between them. So i hope that you will show me that diference. anything that makes the difference between them ( contact position,angle ,playing elbow....) anything that make the difference between them. thank you so much. i'm looking forward to hearing from you.?

Hi Anthony,

The difference is only the slight change of angle with the racket at the point of impact.

This is what makes the serve so difficult to read.  He doesn't give much else away to indicate which way the ball is going to be delivered.