Now, Nintendo explains in the app's update notes that you can both browse to other iPhone apps and lock the device, and voice chat will continue to function in the background through the Nintendo Switch Online app.
The expanded functionality of voice chat should greatly enhance the usefulness of Nintendo's app and online services, which will turn into a monthly paid subscription sometime in 2018. Pricing and tiers include $3.99/month, $7.99/3 months, and $19.99/year.
The app first launched on July 18, a few days ahead of Splatoon 2's debut, allowing gamers to visit SplatNet 2 on their iOS device to look at previous match statistics, check out current stages in rotation, and invite friends to Private Battles on the Switch console. Nintendo's upcoming 45-minute Direct will begin this afternoon at 3 p.m PT and plans to focus on games coming to Switch and 3DS consoles.
Source : https://www.macrumors.com/2017/09/13/nintendo-switch-app-locked/