How to activate the INVISIBLE barrier that appears when you move a clip next to another?

Hello,

I think mine was deactivated by mistake? I can no longer move a clip to another and get that invisible barrier that tells me I am at the level of the clip I am approaching for some reason. Did I deactivate it by mistake?

Its especially useful when you have 2 tracks, and you put one clip on a different track but you want it to start at the edge of another clip, the barrier will appear automatically showing you the edge of the other clip.

Mine has dissapeared and does not show up anymore.