Android P introduces a new power menu 'Lockdown' option which opens up the capability to disable various unlock options, such as the fingerprint unlock, voice match, and smart unlock, though it reportedly works single-use, meaning you would have to enable it each time you want to enter the lockdown mode. With this on if you have a different unlock method set up, such as a PIN, which you would need if you have the fingerprint sensor unlock enabled, you would need to use the PIN to unlock your device after putting it into a lockdown.
Since this only seems to work on a single-use basis, it's not clear if this was the intended result of the feature implementation or if Google will end up changing things for a final release of the software which is supposed to happen some time in the third quarter of the year, likely between July and the end of September. It's important to remember that this is only the first developer preview build of Android P, so there are numerous changes that are likely to take place between now and the final build of the software that Google will push out in late 2018. Keeping that in mind, Google could end up making it a permanent option that will stay enabled until the user turns it off, or Google could simply choose to remove the option altogether before it releases the final build.
The new 'Enter Lockdown' button won't be immediately displayed as you will need to toggle it on. To do this you'll find the option under the lockscreen preferences page located in the settings menu, and halfway down the page you'll see the toggle for showing the lockdown option. Once you toggle this on, anytime you press and hold the power button down to get the list of buttons available on the side of the display, which already included power off, restart, and the new screenshot button which was also added in Android P, the new enter lockdown option will appear. This is just one of many new changes that are likely to be part of the new software, and although it's not a major addition to the operating system, the little touches can end up being the most important to users.