Relays are small electric switches...they act on 6V but I've heard they can be found with inside coils that react on 3V.
You need relays if you want to put way more volt on a motor that the receiver can't handle. If you, for instance, put 6V 2000Mah thru a 3V receiver unit it will burn out the relay does the job of converting the 3V to 6V.
This is the thing I did on the radio shack RC car to make it hop.
What I said before it can be done 1/24 scale but you need a big model car so you can stash all the components in it that's why a van or a Suv is the best for this job.
