Sorry, but no, the "shift" is in the way the transmitter sends the signal, specifically which way the frequency is "shifted" from center to encode the data.

On our FM and FM/PCM radios, the transmitter actually broadcasts on 2 frequencies, the "center" frequency, and another one 5khz away. Positive shift transmitters use the center frequency and another one 5khz above the center, while negative shift uses a frequency 5khz below center.

For example, channel 12 is 72.030Mhz. That's the center. A positive shift TX on channel 12 will be putting out signal on 72.030Mhz and 72.035Mhz, while a negative shift TX will be using 72.030Mhz and 72.025Mhz.

As noted, some receivers will check both above and below the center frequency to see which one the transmitter is using.

Exactly how the transmitters are using both frequencies is detailed here, which is a great article about how our TXs and RXs work (not including the new 2.4Ghz systems):

http://www.torreypinesgulls.org/radios.htm