What is the point of each step in the recipe?

PLEASE read the getting started Q&A before reading this overview, otherwise it won't make sense!

Using 1/2 pint mason jars.
1/2 pint jars are ideal for the PF-tek for various reasons. First and simply, they have been "tried and true" for quite a long while now. But, there are better answers than that, of course. Glass jars have been used for the (edible) mushroom cultivation for quite a while, for spawn production as well as fruiting. Glass jars are easily sterilized, as they can be safely autoclaved/pressure cooked/boiled without damage to the jar, and while maintaining internal water content of the jars. The 1/2 pint size is ideal as well, since jars can "finish" colonizing within 3 -4 weeks. The reason why this is ideal is because you want the substrate to colonize as quickly as possible - so it can ward off any potential contaminants that may settle in the substrate through a vector you have not discovered, through inadequate sterilization of the substrate, etc.
Using 1 pint jars or larger with the PF-tek means the time to colonize is increased to a point where it is no longer worth it when weighing in the added colonization time as well as the additional risk of potential contamination (in its slower colonization times). It is more generally regarded as "the law of diminishing returns." There have been reports of successful PF-tek recipes used in 1 pint jars, but this is the exception and not the norm. Glass also serves a utilitarian purpose - you are able to see the progression of the mycelial growth (and any possible contaminants as well). Glass cups can also be used, but are not as "plus and play" as mason jars are. They also don't seal as well. But, mason jars can be hard to find in some areas, and their substitution can be necessary for the PF-tek.

Using 1/4 cup BRF, 1/2 cup vermiculite, and 60ml water.
To adequately colonize, mycelia needs food, water, air, and proper temperatures. The brown rice flour is the "food", and the vermiculite makes the entire mixture airy enough for air to reach throughout the substrate (and hence, all the mycelia forming in the middle of the cake that you can't see). So why not up the food, and make colonization quicker? Because, too much food can be a bad thing. The "food" which mycelia loves so much also attracts contaminates, and more food means more of a possibility of contamination. And if mycelia likes air, why not increase the amount of vermiculite in the mix, making it more airy? Once you mix a batch, you will see that the recipe as provided is about as "airy" as vermiculite will get in the mixture. Also, the addition of one element will mean less of another. Water content should also be kept as close to the ratio provided as well. Too much water and too little water both can slow the colonization of substrate to a halt. Plus, too much water will make the mix pasty and muddy - so that the mycelia will have a hard time colonizing through it.

Using a dry vermiculite "seal" on top of the substrate.
The vermiculite seal has 2 purposes.
- It stops airborne contaminants from settling on non colonized substrate
- It also acts to provide some air exchange to the jar.

Air exchange can boost the rate at which mycelia grows, so it is important to have some measure of fresh air passively able to reach the substrate. Also, mycelia and mushrooms create CO₂, while using a small amount of O₂. Allowing for some gas exchange can help speed colonization. Many people whose PF jars have slowed "flip" their jars, so that the lid is on the bottom - which allows for more air to circulate to the non colonized bottom of the PF cake (the top part generally colonizes more quickly than the bottom).

Lift the jars up from the bottom of the pot.
The heat from the burners on the stovetop transfers directly to the bottom of the pot, which transfers the concentrated heat to whatever is sitting at the bottom of that pot (your jars). Lifting the jars by placing them on the cooking rack that comes with the pressure cooker, or placing a washcloth on the bottom of the pot, will create a more even disbursement of heat. This means the bottom of your jars won't be "cooked" by the heat emanating from the burner. Remember, you're sterilizing by steaming the jars, not by cooking them. A jar whose bottom has been cooked will oftentimes not colonize on the bottom, as the resulting overcooked cake becomes hard and impenetrable for mycelia. Overcooking can also occur if your water level runs too low, so be sure to maintain water in the pot for the entire sterilization time. AND DO NOT ADD COLD WATER TO THE BOILING JARS, otherwise they will crack and be unusable.