No… if you put a bucket of water in the toilet bowl when you are done it will automatically flush itself and not empty the tank at all.
It has to be enough to reach a certain height in the bowl and then it will just go away with gravity. It is an excellent idea.
Also another idea is to glue pieces of styrofoam to the inside of the toilet tank to make it smaller. That too will allow less water into the tank for flushing with.
The bottle with the sand in it is a great idea as long as it is heavy enough to not fall over. This will disburse the area where water would have been and is a great money saver.
If you are in the process of redecorating your washrooms and replacing the toilet they make water saving toilets now that are a little smaller and this definitely helps to prevent the overuse of water.