Crystal are what keep the transmitter/radio and receiver on the same frequency. If you race or drive your cars with other people and you notice someone is on channel 88 and someone is on channel 71. these channels correspond to the frequency of the crystals.

If someone is on the same channel as you, you cannot run your car. You will be able to control his car and/or he will be able to control yours. That is a bad situation to be in.

You can buy extra crystal on different channels and change them as needed to an open frequency. Better yet you can increase your budget for a new radio and get an Airtronics MX-3 FHSS. They run on 2.4 Ghz and you don't need to change crystals.

In my opinion the Airtroncis MX-3FHSS is better that the Spektrum because it has frequency hopping which searches for the best signal every couple of seconds and it is a pure digital signal meaning faster response and latancy time. The Spektrum finds an open frequency and stays locked into that frequency for the duration of you run or race. This increases interference and glitching especially if some one is using a cell phone with bluetooth or cordless phone or anything else on 2.4 Ghz.

I hope this helps.