If there is a gap between the coffee grounds the pressured water, the water will see this as a "short cut" and flow faster through this area. 

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The tamp should create an even bed of coffee where there is no empty spots or "short cuts".  The pressure you place on your tamper should be around 12 to 13.5 kilograms compressing the coffee evenly, you will also see the guide line on the basket if tamped correctly. You can use this as a guide to ensure level tamping.

 

Your arm should be perpendicular to the surface and the handle should be level as well. If your not tamping evenly (usually caused by a wrong arm angle or the porter filter being on an angle.) the water flowing through will flow faster through the gaps. 

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If your a barista the angle is paramount as you don't want to strain your arm throughout the day. 

If coffee is ground right and you're seeing coffee flow too fast this could mean the tamp was not hard enough or the dose of coffee was to little. If you see a slow flow or "dripping flow" this could mean the opposite. Your dose could be too high or your tamp could be too heavy. 

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A lot of this is easily learnt by repetition once you find the correct tamping pressure you will over time understand the strength of pressure to place on the coffee.