If you start powering down, the entire amount of your SP holdings is divided by 13, and then you receive every week one of these 13 installments in your Steem account. The underlying logic of this was that people have some "skin in the game" and it was tied to the whole Proof Of Stake design across Steemit. But as more people started to use the platform, more use cases emerged.
One of these uses cases was presented to me by @everittdmickey, which, in short, suggested that we could have a variable power down amount. This amount should be no more than the allowed 1/13, obviously. The suggestion makes a lot of sense, because many people don't need their entire amount of their SP, just a smaller portion of this, and not all the time.
As a witness, for instance, I would only need the equivalent of the server costs, no need to start a full power down that will deplete the account in 13 weeks - and with it my voting power. Of course, I could stop power down, keep what I need from the converted amount and re-power up the rest. But it's complicated, time consuming and adds more transactions to the blockchain.
I opened a Github issue here, if you feel this would be beneficial, or if you have other opinions, please go ahead and comment. If we can convince the developers this is a useful feature, it will be included in future upgrades of the network (or hardforks, as we call them here). Be polite and relevant, please.
Thank you.
I'm a serial entrepreneur, blogger and ultrarunner. You can find me mainly on my blog at Dragos Roua where I write about productivity, business, relationships and running. Here on Steemit you may stay updated by following me @dragosroua.

https://steemit.com/~witnesses
If you're new to Steemit, you may find these articles relevant (that's also part of my witness activity to support new members of the platform):