DX lenses project a smaller image circle than FX lenses, smaller than an FX sensor. DX mode allows you to use an area the size of a DX sensor in the center of your FX sensor so the DX lens image circle will cover this area. Since each photosite in this area is still its full FX size, you will retain all the ISO and dynamic range capabilities of an FX sensor. However, since you are only using about an area about 45% of the full FX sensor, you will lose about 55% of your megapixels. This means a 36MP FX camera will produce 16.2MP images in DX mode, and a 24MP FX camera will produce 10.5MP images in DX mode. In addition, in DX mode, you will experience the same crop factor effect on your focal length. This means a 35mm FX lens on an FX body will give the angle of view expected, but a 35mm DX lens on a FX body will give an angle of view approximately like that of a 50mm FX lens on an FX body, just as that lens would do on an DX body.

If you have a large investment in DX lenses, DX mode can be a reasonable bridge, but you will not be using about half of your very expensive pixels, so if you have fully committed to moving from DX to FX, you should seriously consider selling your DX and lenses and acquiring FX lenses to take full advantage of your sensor.