A delay line takes a write and echoes it back N cycles later,
with N fixed at compile time. It's a handy primitive to have
when wrapping Verilog blackbox modules because the blackbox
often specifies something like having 2 cycles of latency,
and so you need to bubble the fact that a write occurred 2
cycles ago through to the output so that you can wire up the
right implicit conditions.
Partial synth is handy when writing gnarly Bluespec modules, because it
lets you inspect the Verilog output of the Bluespec compiler as well as
Yosys's compile output at various stages of synthesis, to see if things
are being produced the way you expect.
David Anderson