Is it me or am I missing something here but - The round bit creates a seal around the plug hole and as you push down on the handle the air contained gets forced down the plughole into the waste pipe and hopefully shifting the blockage.
It will still not work like suggested earlier by almcd007 if you do not hold a wet cloth tightly on the over-flow or the pressure will just ecape through the un-covered overflow and will not go down the U-bend to shift the blockage.