Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
As noted, it can vary from channel to channel. Some ad-supported channels may only allow it to the next ad break, where it will stop and play the ads. Some may have a form of slideshow while using FF or REW to help you see where you are. Those that use the basic controls just show the time bar and you have to guess where to stop.
@andyross Just brought up a good point. Channels with ads might allow FF and rewind during the movie, but not during the commercials. You are forced to let them play. Even when it's the same ad playing 5 times in a row.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.