The Drawbridge is not in the two latest stable versions of TiC, as mDiyo has moved it to the Mechworks mod.
The Drawbridge is a machine that allows for a bridge to be built up to 15 blocks long. It is activated with a redstone current and deactivated by taking away the redstone current. The Drawbridge can also be camouflaged with most every solid block and be placed vertically as well, allowing it to go up or down.
There are also two variants of the Drawbridge: the advanced drawbridge and the extended drawbridge. The advanced drawbridge can place a pattern of blocks such as wool, wool, redstone, obsidian, Diamond, etc. The extended drawbridge can go up to (and past?) a full stack of blocks, making it able to go larger distances.