Strange new sensation

I posted this as reply to an older message and it didn't get much (any) response so I was hoping that by posting it as a new topic might get some feedback. I'm curious to know if anyone has any thoughts for me.

I put the MGX in a closet for the last year and gave it a rest. While I was making some progress, I had such infrequent opportunities to drop out, tune in and turn on that it just got put aside for a while.

Tonight I set aside the entire evening to relax and get back into my training.

I started off slow and by the book, turning off all distractions. I started on my back, moved to my side after about 20 minutes and eventually ended up face-down bouncing my hips off the mattress. The thrust bounce didn't really offer much to my penis but created a motion that was clearly discernable on the prostate. There was some reaction but after a while it was time to change positions again so I went on my back with the intention of trying the SLINGSHOT.

I didn't have the leg strength to try it for long so I ended up with legs in the air and the soles of my feet together. I used this as an alternate and switched between the slingshot and 'feet together' trying to spawn some involuntaries. At some point I felt what I assumed was trapped air escaping past the MGX as I brought my legs down for the slingshot. I was on my back, knees bent and feet flat on the bed when I suddenly felt my glutes shaking. My legs were pretty tired from holding the different positions so I thought it was extreme fatigue. The shaking worked up my thighs and out to my knees and my lower back. I was stunned at the level of shaking and realized that this could be some sort of p-wave or other desirable outcome. When I caught on that this may be a 'good thing,' I started breathing deeply and moving my knees apart and bringing them together slowly. I was able to affect the level of shaking by the position of my legs and if I remained stationary it would eventually abate but I could egg it on by moving my knees one way or the other.

The feeling was a tingle at a minimum but at the top end of the shaking it was like hooking a 24v DC battery to your large leg muscles while you drive down a bad dirt road. I would not describe it as a sexual experience but I had three erections come and go during this event. The more I tried to slow things down and breathe, the better my control.

It lasted for just short of 35 minutes. After all of this I had to get off of my back and rolled to my side and took a break for a few minutes and tried to start things over again. No luck from that point on but I was pretty tired by that point.

I know something happened but what did I do and what should I try next time to help things along? I didn't expect anything to happen after a year off so it was a pleasant surprise.