If you're worried about accidentally throwing your Nintendo Switch on the ground while playing Pokémon Let's Go in public, you don't need to be concerned.
While an undocked version of the Let's Go titles wasn't available at the event, a Pokémon Company representative at the NintendoVSUK event this weekend told The OP exactly how catching Pokémon will work.
While playing the game in handheld mode, the player collides with a Pokémon in the overworld and attempts to capture it. The player must physically turn their Nintendo Switch around to find the Pokémon in a 360 degree space, but you only need to press a button to throw the Pokéball at it.
By what the Pokémon Company representative was describing, the mode seems to work like Pokémon Go's AR mode and the old 3DS eShop title, Pokémon Dream Radar, combined into one capture mechanic.
The news confirms a similar report from Eurogamer last month.
Currently, Nintendo has only shown off the Joy-Con motion control options for the game while in docked mode, making it sound like they would be compulsory for the game. This news, however, seems to offer a different way to catch Pokémon if motion controls aren't you thing.
During a trip to NintendoVSUK event in London over the weekend, Nintendo offered a surprise look at a small tech demo of Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! to press in attendance.
There, members of the press were able to explore Viridian Forest for a few minutes and even discovered how Shiny Pokémon will work in-game when attempting to capture it.