So I always thought that it actually never gave it much thought, that by raising the lever on my Rocket Appartamento, that some built in pre infusion was happening.

I never had spurting it spraying or severe channeling.

Well, I got the flow control device and installed the stiffer spring.

I never partially raised my lever prior to installing the fcd.

So how does pre infusion work on a Rocket Appartamento? Partially raised lever?

How does a stiffer spring eliminate pre infusion?

Thanks.

Raising an E61 brew lever part way doesn't do anything, unless your water supply is under pressure. Then, you'll get pre-infusion at whatever pressure that is. That pressure comes from plumbing in your machine. If your supply line pressure is higher than you want for PI, then you can use the flow control device to restrict it. If your machine isn't plumbed in, then you need to raise the lever to the brew position to activate the pump, and use the flow control device to restrict the pressure to your desired PI value. Once your PI is done, in either case, you'll open up the flow control valve to allow for extraction pressure. If your machine is plumbed, you'll also need to raise the lever to the brew position at that time, so the pump comes on.

So why replace the spring?
With a stiffer spring ?

It's a little complicated, especially on machines like some Rockets that have an inlet solenoid valve. With these, even if plumbed in you won't get line pressure when the lever is at midpoint. What you do get with the lever at midpoint, even if not plumbed in, is a small amount of water from whatever pressure had been built up in the HX as a result of thermal expansion. Wetting maybe, but not really predictable pre-infusion.

The E-61 has two valves and springs below the brew chamber that are forced open by the cam when the lever is fully dropped. The topmost of these leads to a small pre-infusion chamber just above the exhaust port. When the lever is raised and pressure first builds in the brew chamber it will force that valve open and some water will flow into the pre-infusion chamber. It doesn't go any further because of the strong spring on the drain valve at the bottom. Once the pre-infusion chamber fills and pressure that builds inside there equals the brew chamber pressure, that valve is closed by the spring. You can see pictures here: E61 Group Espresso Machine: Detailed Interior Schematics
If you are directly controlling with a flow control valve, it makes sense to just take the pre-infusion chamber out of the loop. With a strong spring it, along with the drain valve, are always closed unless you drop the lever and the cam forces both of them open.

The stiffer spring holds the preinfusion valve closed, like a drain valve so that the preinfusion chamber is taken out of play.
Thus when you raise the lever the whole way and the pump is on, the preinfusion valve isn't "leaking" at 1.6 bar like it would if it had that sofe, easy spring, exposing your prep(the ground coffee) to 1.6 bar until the preinfusion chamber fills.
So. You use your flow control valve to limit the flow to the prep. You raise the lever the whole way. Your pump comes on, your brew valve is fully open because the lever is the whole way up. You have your gicleur almost completely closed off so that only a trickle of water gets to the prep for 5 or 7 seconds giving the prep(the puck of coffee grounds) time to evenly dampen and "bloom", if you watch the gauge you can finesse the flow control to let it up to 1.5 - 2 bar and then close it back to maintain that for a few seconds.
Then when extracting, open the flow control up to maintain the 9 bar (or whatever your imagination suggests) for perhaps 16 seconds, then slowly bring it back down to peter off the flow for the final 6 - 10 seconds.
Try things.

YMMV, but what I decided to use the stock (less stiff) e61 preinfusion spring with my machine that came equipped with an ECM FCD and the stiffer lower chamber spring supplied with it. I preferred this arrangement, which let's you use the "classic" e61 "preinfusion" but also allows you to employ flow control during extraction as you see fit. I believe the stiffer spring causes more wear on the cams than does the stock spring, though I'm not sure how detrimental, if at all, that is for the machine.

Does the Appartamento have pre infusion?

If so, how does one access it?