basically when you get a brand new nitro engine.. they have just come off the production line.. the engines piston and surround and casing have all only just been machined.. this means that they are all a VERY tight fit.. and have not yet ever been heated up.. this means that when you first run your engine it will bend and warp and change in size and wear down any tiny parts that stick out.. etc etc

what this means is that when you first start up a nitro engine you dont want to go mad with it.. as it can cause microfractures and other defects to form in the metal itself.. which is not good.. what you want to be doin is gently running the engine for the first couple of tanks to let all the metal settle into its appropriate positions and if anything is going to warp and change shape.. to let it do it gently to prevent damage..

there are a VAST amount of different methods for doing this.. varying from running the car off the floor at idle for a full tank..
through to what people call heat cycling.. run it gently.. let it cool.. run it.. let it cool etc..

in my personal experience for break in.. i generally just take a car out.. gently drive it around and no more than a tiny bit of throttle.. just cruise it around for a little while and let the engine heat up a bit.. then after youv done that for a while.. let it cool completely.. then next time.. do the same.. but this time maybe give it a couple of short 1/4 throttle runs get a bit of speed up .. again cruise around with the odd short bit of speed...

do this about 4 or 5 times untill eventually your going full throttle occasionally..

once youc done this and youc had it at full throttle a few times.. your pretty much done with the break in.. and next time you take it out.. you can be more aggressive with it.. and start tuning it (as during all this you will have been running it rich) as per the intructions..