Lets see, unscrew the plu, screw the whistle in, turn the engine over AND YES TO GET TDC YOU STILL HAVE TO LOOK AT THE TIMING MARKS.... (we had one of these with a SUN scope in the dealership I worked at in the 60's).

You do the same thing by taking the plug out and just using a finger over the plug hole to tell when the piston is coming up on TDC (and still look at the timing marks). Saves you the steps of screwing in and remonving the whistle, and $22.99 plus shipping.