1,4-Diphenyl-1,3-butadiene is a chemical compound that's part of the diene family, which often sees usage in organic synthesis and materials science, particularly in polymers and dyes. Toxicity information for this specific compound might not be as widely studied or readily available as it is for more common industrial chemicals. Generally, the toxicity of such organic compounds can depend on multiple factors including the route of exposure (inhalation, ingestion, or skin contact), the dose, and the duration of exposure. It's advisable to handle it with the standard laboratory precautions, such as wearing protective gloves, goggles, and working under a fume hood. For exact toxicity levels, consulting Material Safety Data Sheets (MSDS) or databases from regulatory agencies such as the EPA or ECHA would be essential. In absence of specific data, it's prudent to treat it as potentially hazardous and minimize exposure.

An emulsion sauce is a stable mixture of two immiscible liquids, such as oil and water, which are brought together with the aid of an emulsifier - commonly egg yolk, mustard, or garlic for culinary purposes. These sauces are a cornerstone in cooking, creating a velvety consistency and enhancing flavors in dishes. Examples include mayonnaise, hollandaise, and vinaigrettes. The key to a successful emulsion sauce is the gradual addition of oil to the aqueous phase while whisking vigorously, which disperses and suspends tiny droplets of one liquid through the other, prevented from coalescing by the emulsifier. Mastering the technique offers endless possibilities for customizing flavors in culinary creations.

"Yarn start" is a command used in the context of the Yarn package manager, which is a tool for managing dependencies in JavaScript projects. Primarily associated with projects built using Node.js, this command is often used to kick off the default script defined in the project's "package.json" file under the "scripts" section. Typically, for web applications, "yarn start" initiates a local development server, making the application accessible through a web browser for testing or development purposes. This command's behavior, however, is defined by the developer in the "start" script and can vary based on the specific needs of the project, such as launching tasks for compiling code or automating routine development tasks.