It's called a traveling steady. For large work this is a frame that is attached to the carriage and carries two adjustable arms that are set to keep the work from deflecting away from the tool. If you type "travelling steady" into Google Images, you'll get lots of pictures showing what they look like and how they are deployed in use.
The idea is that the travelling steady, as its name implies, travels with the tool, unlike the more common fixed steady which remains stationary. This means that the support of the work is always close to where the tool is cutting.
The conventional travelling steady is way too large and cumbersome to be used for tiny model engine work. Nevertheless, the idea is a good one and model engineers have come up with many variants that better suit their purposes.
One of these is a piece of stock with a hole the size of the parent stock drilled through it. This is clamped to the left side of the lathe tool with the stock to be cut passing through the hole. With the stock supported right next to where the cutting action is taking place deflection is vastly reduced.
Of course, this simple version has a major drawback. If you want to take a second pass on your cut, the hole in the steady will be too big. So this version is really only good for a cut that can be completed in one pass. Often, on tiny engine parts, this is possible.
When it isn't possible, tiny adjustable fingers [Sometimes a single adjustable finger with a birdmouth "V" groove is used.] are added to the steady so it can mimic the action of its bigger brother with the difference that it is attached to the tool or toolpost rather than the carriage. A Google search of the various fora that support model engine building will probably produce some designs (but it's probably more fun to design your own).
Keep this idea in your mental tool idea repository. Even if you don't need it at the moment, it may come in handy in the future.