There is always the question....."Am I riding or shifting in the correct gears?" Well, this chart will give you more insight in answering this common question.
Here is the proper chainring/freewheel cassette cog combination while riding any terrain without putting undue stress on the chain, chainring & cog teeth, and quite possibly snapping (breaking) the chain under heavy torque.
1) Chainring: the sprocket(s) where the crank and bottom bracket are. Usually there are three chainrings, but if a bash guard is attached replacing the larger of the three chainrings (chainring 3), then there will only be two chainrings (chainring 1 & 2 or other words known as the lower and middle chainring). If a chainguide is added along with the bash-guard, then the chainguide will replace the smaller chainring (chainring 1). Leaving only the middle chainring or other words a single chainring. The most common of the middle chainrings are the 32T, 34T, 36T, & 38T chainrings. Most DH sleds with a single chainring use a 38T or a 40T chainring for a more concentrated cranking power which delivers more speed.
2) Freewheel Cassette Cogs: these are the individual sprockets that make up the rear freewheel cassette where your rear derailleur is. Usually in today's times these freewheel cassettes are 9-speed cassettes, but there are also 8-speed (as depicted in the chart below) cassettes and 7-speed cassettes as well.
Below is for a 9-speed cassette (8-speed depicted above in chart)
If you are in chainring one (smallest) you will only have the usage of cogs 1 through 6 (from the largest moving down), the upper 6 cogs. Anymore than specified, you are putting undue stress on the chain and cog & chainring teeth, and quite possibly you will snap your chain under heavy torque.
In the middle chainring you will have full usage/range of all 9 cogs (8 for 8-speed / 7 for 7-speed) without putting any undue stress on the chain, cog & chainring teeth under heavy torque.
In chainring three (largest) you will only have the usage of cogs 9 through 6 (from the smallest moving up), the lower 6 cogs. Again, anymore than specified, you are putting undue stress on the chain, cog & chainring teeth, and quite possibly you will snap your chain under heavy torque.
On another note:
Power shifting up while in a climb causes undue stress on your chain, chainring & cog teeth, and can cause the chain to snap (break) under heavy torque. Shift into a good climbing gear when approaching the climb, and if needed, down shift to a lower gear to ease the strain on the drive train. It is far better to spin your way up than it is to mash your way up to the top. In some cases, you can blow a knee out by mashing to much. It is also far better to shift in a fluent manner than it is to wildly shift. When you hear all that loud popping and snapping of your chain while shifting, either you are shifting incorrectly or your drive train needs to be adjusted or even serviced. When your drive train is correctly adjusted & maintained and you are shifting correctly, your shifting will be precise and fluent.