Long story short, it's fantastic.
The principle is that you choose a %age of your salary to sacrifice and then all calculations based on your base salary as far as tax is concerned use this new base salary as the starting point. So you pay less PAYE, NI, student loan if you have one. Note that I specify as far as tax is concerned, your employer should however still be using your gross salary as the basis for all pay rise calculations, so there's literally no way you lose out here, other than the fact you get less cash in your pocket right now. You get more net worth overall, it goes direct to a pension pot and, really helpfully for a lot of people (and I'm hoping for me in the next few years), you can use your SS contributions to keep you out of the higher-rate tax bracket as the tax brackets are based on your 'adjusted net income' i.e. your income after these SS pension deductions have been made.
I know, for example, another mustachian who has been increasing his SS % every year as he gets pay rises to maintain his salary at about £45k post-SS so he never hits the higher bracket (and also maximising the very generous pension contributions from his employer). It means there's a potential he's going to fall foul of the £40k max annual contribution to a pension in the future, but like all good mustachians he's aiming to have retired by then and therefore will not need to worry about it!
In the UK your employer is also almost certainly legally bound to contribute due to the auto-enrolment scheme (details here:
https://www.gov.uk/employers-workplace-pensions-rules) which means that not only are you getting to put a gross salary amount in and therefore not paying any tax on that amount, but your employer is ALSO putting into it and that's not being taxed either! This is how, on a few occasions in the last year, I've managed to make it to savings rates over 100% (I base my savings calcs on take home pay) as we've spent a smaller amount on our monthly bills than my company is contributing to my SS pension...