I cannot agree more. After some time in your cycling classes, you may start to develop enough conditioning that a regular class seems very manageable. Do not become tempted to become content with your workout (even if you are following the program). If you're not pushing yourself, you're simply doing maintenance work and will likely not see any results.
However, you may notice that there is usually a break between songs the instructor uses for delivering instructions. Use this time to continue with the same intensity from the previous song and continue working your buns off.
Imagine this like a soccer game, which I'm sure you've played before(?). The common saying is that "soccer players do it for 90 minutes." But really, if you've played soccer, you realize that you aren't actually RUNNING for 90 minutes, there is a lot of jogging, standing, and waiting that is involved. Now, if you are ballsy enough, try to hit the track, and spend 90 consecutive minutes RUNNING (not jogging). The intensity and duration of time will undoubtedly have your legs crying for a break.
So the point is, that is all about intensity and pushing that lactate threshold if you want to see some incredible gains. 👍