Assuming you want to invest $1000, you can start investing $50 every week. That is the meaning of DCA. As you are investing $50 as bitcoin decrease further, you will still be able to invest some amount when it's price is low. If you take the average of the weekly invested amount, you may invest at $17000 in average which is better than investing all at $20000.
Well, this was about trading, not investing.
I think that in trading the DCA works in a way like:
* you buy at 20k and set 22k as selling target
* if it falls to 19k then buy more
* if it falls to 18k then buy even more
* if it falls to 17k then buy even more, if you still have funds
* if it grows you sit and wait until the 22k target is met (or, if you've bought at 17k or 15k you can decrease the target too, according to the prices and amounts you've bought at so you get some profit)
I am not sure 100% though if I've got it right.