What makes it a mulching mower?

[Skippydiesel]

Gentlemen:
More Power:
Retired now but whole career in agriculture. Doesn't matter what you are processing, the more you process (in this case render the grass clippings smaller) the more power is required - simple physics.
No matter the size of your machine (push or ride on), an identical one, without mulching kit, will use less power than the one that has it. This can be translated in to less fuel burnt per unit area (acres or hectares) or less time to cut.
Fertilizer:
I have never removed grass clippings from my lawns - thats what worms/fungi/insects are for (incorporating organic matter). Removing grass clippings is much the same process as harvesting/grazing a crop - by doing this you are stripping nutrients from the soil, leading to nutrient deficiency. If you want to maintain a health grass cover/lawn, you will have to replace the nutrients lost, by fertilising (chemical or organic). In my view a crazy notion.
Blades Cut Multiple Times:
There will always be a degree of material "churn" within the cutting cavity - its just not possible to eject each cut grass clipping without some recut However this is not the intention. If your mower deck has been correctly adjusted, there will be minimal recut/processing.
Think of the blade as a disc, with one side, half of the disc, being the forward/leading edge (cutting arc). For most effective, least power required, operation, the disc should be set so as to achieve a leading edge down angle ie as the leading edge leaves the cut sward, so its trailing edge (arc) should be fractionally above the cut material. No additional cut or drag, so lowering hp requirement.

 

[bertsmobile1]

Mulching does require more power but not because you are cutting the clippings multiple times
It requires more power because you are pumping more air, a lot more air to keep those clipping airbourne .
This assumes you are not trying to mulch 2' tall grass

 

[Skippydiesel]

Maaaate!

A conventional mower deck is similar to a centrifugal pump/aircraft propeller. Cut off the air supply/exit (as in a mulching mower exit plug) you use less power and move less air.

Most mowers use blades with a "pitch"/twist so as to act as fan/propeller. Propellers are just a type of (air) pump. The fan function is two fold. Air is drawn in, usually from the underside of the deck "skirt" to lift the sward, for the blade leading edge to cut. The combination of air and clippings, plus acceleration imparted by the blades on the clippings, is then used to blown the cut material out of the the exit shoot.

My limited understanding of how a mulching mower works is:

• Fitted with blades designed to enhance the break down (mulch/render) of the grass clippings, by additional impact (cutting) and then force the clippings into the sward/lawn.
• Exit shoot is usually blocked, so that the clippings will be retained within the deck cavity, thereby facilitating the additional processing of the clippings.
• The blocking of the exit shoot reduces the effectiveness of the fan function (blocks exit air) - to try & counter this, mulching blades may have courser pitch blades.
• The concept of a mulching mower, is that extra fine clippings created, will be forced into the sward for more rapid decomposition/incorporation (little evidence), unsightly (to some) grass swath may be eliminated and for those inclined to removing grass clippings, this action will not be required.

Doesn't matter what a machine is processing (grass/grain/gravel/etc) - the more its processed, for a given throughput, the more energy is required.

Reduce the throughput (slow the mower &/or raise the deck) and reduce the energy/hp needed & visa versa.

It's called physics and even Einstein worked within its laws.