Call-to-action (CTA) overlays are compatible with:
- TrueView in-stream ads only
- TrueView in-display ads only
- TrueView in-display ads that are at least 30 seconds long
- any TrueView ad format
CTA overlays are available for TrueView in-stream ads or in-display ads. Once someone clicks an in-display ad to watch the video, the CTA shows on the watch page. The CTA overlay appears as soon as the video starts playing, and then collapses to show just the thumbnail image after 15 seconds. On desktop browsers, viewers can hover to expand the CTA. On mobile browsers, viewers can click the thumbnail image or arrow to expand it.
Read more here: https://support.google.com/adwordspolicy/answer/2679940?hl=en