What exactly does the power on/off button on a Dolphin 13s pack do? I know it shuts down any output and I assume it does this thru the BMS,

Specifically, I plan to have 2 Dolphin packs (same voltage) wired such that they can be in parallel on one of my bikes. In the following scenario:
- I have one of them (the secondary one) unplugged and then "use up" the primary while riding.
- I then power off (but leave physically connected) the primary and plug in and power on the secondary - both physically still in parallel
- In this scenario, is there any danger to the powered off primary pack, by say a rapid discharge into it from the secondary?
- Or does the fact it is "powered off" prevent the discharge ports on it from seeing anything?

I personally would have a 3way switch, run A or B separately all the time, or A & B together, all the time...

I agree with 350HTR. There's nothing wrong with your theory, but the reality is that the power button on a Dolphin can be flakey.

If you think you pressed the button to power off, but it didn't activate, there will be big currents flowing from the full pack into the depleted pack, and it's not like it is coming in via the charging port.

A three way switch is a good idea, but there is still a few concerns. First the switch must break the connection from one battery before connecting to the other. Then its contacts have to handle the current from a hot switch. Unless you know you have both under control, adopt the habit of making the switch with both Dolphins off. Then you power up the active one.

OK, make it a 4 way switch,

1; off
2; a
3; b
4 a & b...