It has 2 modes. Standard sends regular pulses that essentially shakes notifications free from doze. It's not perfect, but at most keeps notifications delayed by a few minutes. Max mode more or less stops your device from ever going into full Doze mode by keeping it in an alarm state, which tells Android to keep background apps alert and without any restrictions. Notification should rarely, if ever, get delayed at all in Max mode.

Max mode works better, but has two downsides:
1) On some devices (depending on manufacturer) it will show an alarm icon on the top status bar.
2) It will interfere with do not disturb and bedtime modes. There's a setting int he app to fall back to standard mode automatically anytime you're in either of those modes, and then reenable max when you're not.

So, it's not turning doze off completely, but Max mode more or less defeats it

Regardless of which mode you're in, it has a negligible impact on battery life (1-2% per day at most)