Lets say you plan to do 15 grams of E (dry E) so you put it in a in a 1 liter flask ,

you decide you want a extremely fast reaction,

Add 1.5 x I2 or 22.5 grams of I2 (dry I2). That is an excess and will insure plenty of I2.

Add 0.8 x Rp  or 12 grams (dry RP) This excess amount of RP will insure that PI3 and I- are available very rapidly to the reaction as it is needed.
Mix it together and it sits there.
Tape on a stopper with an attached fume hose and inject 4 mls of water into the hose so it drips into the reaction flask , shake it up and 'viola' a spontaneous reaction , no heat is added , the reaxtion is very strong bubbling seething producing gas.

Shake it and chill it obtaining a general control and slowing the reaction,
A little heat is added and the reaction fires off hard turning darker and turbulant, within 30 seconds the redness seems to vanish the reaction slows and the walls of the flask become clear/

Add another blast of heat  and inject another 2 ml of water, the reaction bubbles and fizzes slightly indicating it is basically dead, the whole reaction took place in 5-10 minutes and produced 10 grams of some excellent speed.

Had  10-15 ml of water been added before the RP, the reaction would not fire spontaneously.
but you could reflux it for several hours and it would eventually turn ,
you could lightly boil it in an open flask releasing water and HI until it thickened 1-2 hours and you would have meth

the reaction mechanism is the same.

Individual steps
---------------
P4 + 6 I2 -> 4 PI3
4PI3 +4 I2 ->4 PI5
4PI5 + 16 H2O -> 4 H3PO4 + 20 HI

10 E-OH + 20 HI <-> 10  E-I + 10 H2O + 10 H I
10 E-I +  10 HI  -->> 10 M-H + 10 I2

Overall reaction
------------------
P4  + 10 I2 + 16 H20 + 10 E-OH --> 10 M-H  + 10 I2  + 4 H3PO4  +  10 H20
not that simple is it

Where
P4 = red phosphorous
I2 =  crystal iodine
E-OH= ephedrine
E-I = iodated Ephedrine or iodated  pre meth
M-H= methamphet.

Water is used up
P4 is turned into phosphate and lost
Iodine is restored  (the actual catalyst) except it is part of the reaction.

What can be said about what promotes and what retards the reaction
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More P4 creates more HI
More HI pushes the reaction to the right
P4 absorbs water pushing the reaction to the right

So the Phos uses whatever water it can
find to make HI from I2
And pushes the reaction to completion.
 

So What??
Who cares about  that stuff ?

To get the reaction to fail do this:
-----------------------------------
Too much water, or too little water
Too little Phos
Too little HI or I2

A sucessful reaction has this:
--------------------------------
Just the right amount of water
Slight excess of Iodine
Lots of Phosphorus

For those who wish to exercise control over the rate of the reaction:
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The rate of the reaction can be controled by the amount of water it is supplied. By adding water a little at a time, the reaction rate can be increased or decreased
 or by
The amount of Phosphorous added
or by the amount of iodine added

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