Exactly that, the “bed” is the selling your investments, the money is then transferred to your ISA, and then your investments are bought back.
The £20k annual allowance still applies, so if it exceeds that the remainder should go to your general investing account again.
It’s basically a way to save you manually selling, transferring the cash and rebuying which at legacy would be costing you £10+ a pop on each of the sells and buys, whilst this is a one transaction cost and minimises the impact in changes in the market.
Edit: I should add its not a get out of jail card for paying CGT. So may be worth manually selling some if you are going to be making more than £12k gain. And then waiting a week and sell some more in the new 2020-2021 year (£12,300).