Switches back to 2D mode. You can use any 2D command while in 3D mode but the results of most (e.g. PRINT and DRAWSPRITE) will look extremely strange.
Be aware that you can use any 3D command while in 2D mode, but you are likely to get very strange results from most of them (they will be drawn on an orthogonal screen). It is advisable to switch back to 3D mode before using any 3D commands. You can switch back and forth as many times as you like before calling SHOWSCREEN
See 3D tutorials.