Why the transition effect can not previewed in timeline?

Whole operations in YouTube: https://www.youtube.com/watch?v=7yTyiInSJMY

Is there any option can toggle preview effect of transition?

Hi,

Moving the play-head is designed for editing purposes (selecting range, cut, trim, etc), you should click the preview button (as you did in the beginning of video) for viewing the transition & animation effects.

Regards,