Yes, you need to have the kite powered as much as you can comfortably handle... If you have too much power you will get pulled off your edge before you can pop off the water. I have the kite trimmed to be ridden comfortably when the chicken is about 3 inches from the bar, then when I am ready to unhook I get a surge in the kite and use that to load my pop and boom! haha!.. I am riding hadlows though and this works well for this kite..
Just remember ride the pop up before you open up for the railey, as soon as you open up for the railey you will stop going up and start going sideways so ride the pop to the top then open it up for the railey and they will feel amazing..
as far as back lines being slack, I dont have any slack, not sure about the switchy's but if you depowering your kite that much to unhook I would tend to rig down to a smaller kite so you have controllable power not lots of power..

Hope this helps..

Trying to hold down a fully powered Switchblade unhooked is going to make the whole thing a hell of a lot harder... They are set up to oversheet when fully powered so you can boost better.

On the 12m and 10m Switchblades, pull about half your depower strap on before you unhook. On the smaller ones pull about a quarter of the depower strap on.

If you're already really powered, pull the whole strap on - it's going to give you a bit of slack once you pop so you can eventually pass the bar - plus it won't rip your arms out....

Hadlows or C kites are different - you just load 'em up and pop, the trim doesn't matter so much...

Unfortunately that is one problem ( having to adjust the strap quite often when unhooking ) with the SBs particularly in powered conditions and it lead to many of the trim straps eventually wearing through, it hasn't happened on any my newer SBs but I replaced quite a few on my SB 2s and 3s. You can find a happy medium but it's not always perfect.