"Pulling" a pin low
 

Does this mean to short it out?

For instance, we've currently got our maxbotix sonar in constant polling mode. If I wanted it only to check distance on command, would I hook up the RX pin to a digital output, or a relay, and turn it on?

I'm a little confused about the terminology.

http://www.maxbotix.com/uploads/LV-M...-Datasheet.pdf

Re: "Pulling" a pin low
 

It'd be preferable if you shorted the pin to ground through some non-zero resistance, but that would be one way. The datasheet actually says to hold the pin low to disable ranging, which is a slightly different concept from a pin being pulled. To "hold" a pin low or high, you apply either GND/Vss/low voltage or Vcc/high voltage to it, depending on labeling conventions, etc. So the only requirement to stop the ranging is to apply 0V to the RX pin in whatever fashion you choose. You could simply hook a digital output to the RX line and set it to 0 when you want ranging off. Using a spike would be more complicated and would also necessitate using 12V instead of 5V as the high voltage which would probably fry things.

Re: "Pulling" a pin low
 

Holding a pin low should mean connecting it to ground. You should be able to do so by hooking the pin up to a digital IO in output mode and setting it to 0 when you don't want the sensor to run.

(Granted, I've never actually used the digital IO as outputs, but that's what should work how I understand it.)