Unix pipes have built in back pressure. A very simplified view, assuming blocking calls and single threads: if the process on the left side of the pipe produces data too fast and fills up the buffer, the operating system won’t give it any additional CPU cycles, until the program on the right side of the pipe caught up with reading and freed up the buffer.

Things become more complicated when multi-threading and / or asynchronous IO gets involved. If the producer on the left side of the pipe has multiple threads, e.g., one writer thread and multiple worker threads, producing data for the writer thread. Then it has to invent its own signaling between the different threads, to avoid throwing away data, when its internal buffers fill up. This is effectively a kind of back pressure.

In your example with the lever and the parser: if the lexer is multi threaded, you either need unlimited buffer for the tokens or some signaling for the threads, to slow down when the parser cannot keep up. This example is a bit artificial, since most languages don’t allow parallel lexers. But with parallel compilers and one (incremental) linker, this becomes more realistic.