To record a tab, click on the icon in your browser and a window appears to access settings and start/pause/stop recording. To learn more about how to use Screencatstify, check out their YouTube video.
I had some issues with it dropping audio and frames and had to change the setting to 1FPS to record the following screencast and it wouldn't let me direct upload to YouTube. Now I was using this on my home WiFi network which is limited in bandwidth, so I wonder if I would encounter the same issues at school on a better network, and I have 2 different Google log ins (one for school & one for home), so I wonder if the multiple log ins contributed to not being able to upload to YouTube. I was able to save the file to my Chromebook and then upload it manually to YouTube without issue.
Overall, I can't complain, and since Screencastify is still in Beta, I anticipate that any issues I have will be resolved with time as the developers receive feedback. I've waited this long, so I can be patient for just a bit longer.