I've tried a couple of times in the past to manipulate the brew pressure and pre-infusion time on my VBM DD.
Today, I seemed to have a sort of eureka moment, but I'm not sure what to make of it.

I decided I wanted to play with declining pressure on my VBM this morning. So, I started the shot and waited for about 10g in the cup before I cut the power to the machine. The machine continues to brew with declining pressure since the pump is off and with whatever water is remaining under pressure. The machine isn't pushing water into the chamber any longer. This is something I've seen suggested in the past for a way to kinda play with declining brew pressure on an e61 box.

My moment of clarity came when I tasted the shot that I brewed with cutting the power to the machine. It lacked the level of bitter of the previous shot. Both shots were run to a specific weight at same dose and temperature. I just couldn't believe that the little bitter that bit the sides of the tongue before was completely gone with the second shot. It definitely made the espresso more enjoyable. Both were pulled 100% ristretto (18g). But, I'm really wondering whether the 'cutting the power' process could have made this impact on the shot and is this non-existent bitterness a characteristic of commercial lever profiles?

Has anyone else played with cutting the power to their e61 lever machines? On a related note, does anyone think this process will negatively impact the machine over time?

I have a very different machine, but I also find a decline in pressure at the end is beneficial.

This is easy to do with lowering the lever until the pump stops and the valve doesn't open discharging the pressure. I stop a lot of shots like this, but it's late in the shot, otherwise it will depressurize too quickly. Repeatable as well.

I was also curious about a declining pressure profile during a shot and wanted to try this on my VBM. I experimented with an easy-adjust, secondary OPV but that didn't give a smooth, easy pressure control. I ended up installing a solenoid that lets me reduce the power to the pump and that reduces the pressure.

I've found the same reduction in bitterness that you did. I reduce the brew pressure by about 1.5 bar after the first 1/3 of the shot has been brewed. I saw this suggested in one of the pressure profiling threads. It reduces the flow of the shot. I prefer the shots made this way because it increases the caramel notes I prefer.

bostonbuzz wrote:This is easy to do with lowering the lever until the pump stops and the valve doesn't open discharging the pressure. I stop a lot of shots like this, but it's late in the shot, otherwise it will depressurize too quickly. Repeatable as well.

I also move the lever to the middle detente (pump off, valve closed) at the end of all of my shots.

So I just experimented with this today on my E-61 HX. I didn't shut the power off like the OP, but I moved the lever to the middle preinfusion position at the end. I noticed a mellower result in the cup.

Will continue to experiment with this technique.