@Matt_C, I think what Richard meant was maximising your capital gains tax limit, which from Richardās post in the UK it is Ā£12 300 , to pay less tax whenever you actually decide to sell.
Lets look at the following example for UK investors based on a capital gains threshold of Ā£12 300:
Lets say you invest Ā£10 000 in buying 10 000 shares of Shell at Ā£1.
Lets say that in a years time the price per share has increased to Ā£4, now the total value of your shares is Ā£40 000 , and you have a unrealised gain of Ā£30 000.
If you sell it all in the same tax year you will pay tax on the profits over Ā£12 300, so you will pay tax on Ā£17 700 (which is Ā£30 000 - Ā£12 300).
However, lets say that you do not currently intend to sell and it is coming to the end of the year and you have not yet used your capital gains allowance of Ā£12 300. You can do what Richard says to use up your allowance and reduce your tax payment in the future by increasing the price at which the tax agency considers you bought the shares.
If you sell part of the shares and rebuy them again within the same month, this does not count as a transaction and the price at which you bought stays the same. However, from what I can understand from Richards post:
Lets say you decide to use up your capital gains allowance, then you could:
Buy an additional 4000 shares of Shell before the end of the tax year, lets say on the 1st of March at the current price of Ā£4 (which is an investment of Ā£16 000).
If you then on the next day sell 4000 shares from your initial pot, at a price of Ā£4 for a total value of Ā£16 000, this gives you a capital gain (profit) of Ā£12 000, considering that you bought the shares originally at Ā£1. Which you can count towards your capital gain threshold of Ā£12 300.
In summary, from what I understand from Richards post after doing this, you would still have 10 000 shares, but your average price (used by the tax authorities / HMRC) would be Ā£2.2. So your unrealised gain is no longer Ā£30 000, but it is Ā£18 000 and it has not cost you a penny (as long as you didnāt have any other capital gains).
This is beneficial, because for example if you decide to sell for the same price of Ā£4 in in a future tax year, you will only have to pay capital gains on a profit of Ā£ 18 000 which comes from multiplying your 10 000 shares by your profit (current price of Ā£4 minus ānew averageā price of Ā£2.2). Yet you still have your Ā£12 300 allowace for that year, significantly reducing your tax liabilities.
Please let me know if there is anything that is still not clear. I may have gotten a bit carried away and made it a bit too complicated.
Note: This is not advice, it is just a clarification based on my understanding of the previous post 
.