According to block~ help, if you bang [switch~] it runs one block of DSP computations, which is useful for performing computations that are more easily expressed as audio processing. Something I read (which I can't find now) left me with the impression that it runs faster than normal audio computations, i.e. as if it were in control domain. Here are some tests that confirm it, I think: switch~ bang how fast.pd
The key to this test is that all of the bangs sequenced by [t b b b b] run in the same gap between audio block computations. When [switch~] is banged, [osc~] fills array1, but you can see that element 63 of array1 changes after [switch~] is banged. Furthermore, no logical time has elapsed. So it appears that one block of audio processing has occurred between normal audio blocks. [bang~] outputs when that accelerated audio block processing is complete.
This next test takes things further and bangs [switch~] 10 times at control rate. Still, no logical time elapses, and [bang~] only outputs when all 10 bangs of [switch~] are complete. [rzero_rev~ 0] is just an arcane way of delaying by one sample, so this patch rotates the contents of array1 10 samples to the right. switch~ bang how fast2.pd
(There are better ways to rotate a table than this, but I just needed something to test with. Plus I never pass up a chance to use [rzero_rev~ 0] )
Finally, I've seen some code that sends a 1 to [switch~] and then sends 0 after one block of processing. In this test you can see that one block of audio is processed in one block of logical time, i.e. the normal way. switch~ bang how fast3.pd
But that second test suggests how you could embed arbitrary offline audio processing in a patch that's not being run with Pd's -batch flag or fast-forwarded with the fast-forward message introduced in Pd 0.51-1. Maybe it's an answer to two questions I've seen posted here: Offline analysis on a song and Insant pitch shift. Here's a patch that writes 20s of 440 Hz to a file as fast as possible (adapted from @solipp's patch for the first topic). You just compute how many blocks you need and bang away. write440File.zip
Here's another that computes the real FFT of an audio file as fast as possible: loadFFT.zip
But as with any control rate processing, if you try to do too much this way, Pd will fall behind in normal audio processing and stutter (e.g. listen to the output while running that last patch on a >1 minute file). So no free lunch, just a little subsidy.