You can use the power adapter of Yoga 900 to charge your phone via a USB cable compatible with your phone. The power adapter will automatically recognize that you plugged in a USB cable instead of the AC power cable as those cables are differently designed.
If you connect a USB cable to the Yoga 900 power adapter the resulting output voltage will be 5V and the maximum amperage will be 2A which corresponds to 2000mA. Consequently, the power adapter can supply your phone with a maximum wattage of 10W (5V x 2A = 10W).
You can only use the Yoga 900 power adapter to charge phones or other devices that have a prescribed charging voltage of exactly 5V. If your phone or other device have a higher or lower voltage you should NOT use the power adapter as it could damage the battery.
If the voltage matches with your device's charger, you can use the power adapter irrespective of the level of amperage. The 2000mA is only the maximum current available and the device being charged can pull the amount of amperes it requires, e.g. it can pull only 1000mA.
Finally, if your device is capable to charge at 2000mA it will charge about 2 times as fast as with a charger that provides only 1000mA. However, as already stated the 2000mA is only the maximum current possible and the actual current will depend on the device.