When you make a personal pension contribution, it is made net of basic rate (20%) tax relief. For example, if you make a contribution of £8,000, HMRC will top it up to £10,000 by paying £2,000 to the pension plan provider. If you are a 40% or 45% taxpayer, you are entitled to tax relief at your top rate of tax for pension contributions.
As you have had 20% tax relief at source, you are entitled to another 20% if you pay tax at 40%. The gross contribution of £10,000 is added to the basic rate tax band which is £31,785 for 2015/16. If it wasn't added to the 20% tax band and was simply deducted from your income, you would get a further 40% tax relief on top of the 20% you have had already.
When you add the £10,600 personal allowance, income up to £42,385 will not be taxed at 40%. Assuming your income exceeds the personal allowance, the pension contribution of £10,000 is added to the 20% tax band of £31,785 so that it becomes £41,785. That effectively drags back £10,000 of income into the 20% tax band which would otherwise have been taxable at 40%.
You cannot get tax relief on a gross pension contribution of more than the lower of 100% of your gross salary or £40,000 in any one tax year. £40,000 is the maximum annual allowance. You can, however, use any unused part of the annual allowance from the three previous tax years. Take a look here and here for information on the annual allowance.
If the salary was £75,000 and a £10,000 pension contribution was made, the taxabkle income would be £64,400 (£75,000 - £10,600). The first £41,785 (£31,785 + £10,000) would be taxed at 20% and the balance of £22,615 would be taxed at 40%. The first £11,100 of the £100,000 gain would be tax free leaving a net taxable gain of £88,900. You would pay CGT at 28% on the whole of the net taxable gain as the extended basic rate tax band has been aborbed by the taxable income.
I hope this helps but let me know if you have any further questions.