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An engine has a "zone" where it is happy and efficient. When you stomp the throttle at low engine speed it is known as "lugging the engine" which is very bad.
What happens is the ECU greatly reduces power by increasing fuel and retarding the timing. It is less controlled because at WOT the ECU enters what is called a power enrichment mode. Typically this involves temporarily entering open loop mode taking the O2 sensors out of the fuel calculation equations. Power enrichment mode is not desirable when you have fuel economy in mind not only because you are using large amounts of fuel to travel the same distance but since the ECU is ignoring the O2 sensors the engine typically ventures into a rich burning state which can be bad for plugs, cat converters, etc.
You aren't really making usable power from the extra fuel but the ECU is making effort to control engine damage... Dumping mass amounts of fuel in and retarding the timing helps the engine lug along until it reaches a point where it is happy again. Once the engine is happy i.e. reaches an appropriate RPM it will add timing and begin to actually make power.
On a side note... 6th gear is an overdrive gear it really isn't meant as an acceleration gear it is meant to be economical.
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