Right, Woland is mainly right (I remember that old thread btw, it was fucking mental). Amortisation is exactly like depreciation, except it applies to 'intangible' assets like player registrations or copyrights, rather than somehting physical like a car or machine.
The purpose of writing assets down is to try to match expenses to revenues in terms of which period they arise in. So if you think a £20k machine is going to have a useful business life of 4 years then you spread the cost over 4 years. This is better because the revenues it helps to generate will obv occur over those 4 years as well, and so for each reporting period you get a much better overall feeling for the underlying profitability of the company than you would by expensing the £20k all in year one.
So a £20m player signed on a 5 yr deal will therefore be written down over that period, with a £4m charge each year under 'amortisation' (this appears in the P&L). Without wanting to get too technical, the 'other side' of the entry (the credit) goes on balance sheet under accumulated depreciation and this gradually reduces the value of the asset stated on said balance sheet.
When a player is sold, 2 things need to be accounted for (the 'money' received, and the asset on the balance sheet still showing but actually sold). The 'money' is credited to the P&L and the NBV of the asset is cleared off the balance sheet and the difference shows up on the P&L as profit/loss on player sales.
Oh, and where Woland is wrong is that amortisation has fuck all to do with tax. It's not actually allowable for tax. Instead HMRC gives capital allowances on certain types of assets, but I'm fucked if I know what how they treat player registrations!!