Beep - that paragraph is about different things. I've pasted what they've said below and then added the (hopefully!) plain English explanation!
"What happens if I go on maternity leave?
This is a 'lifestyle change' and you can decide either to leave the scheme and stop receiving childcare vouchers, or to change the value of vouchers you receive."
This is just explaining that people who don't want the vouchers any more - for example because their older children won't be in childcare whilst they are off- can stop them.
"If you are entitled to Statutory Maternity Pay (SMP), you may choose to leave the scheme before the 8 weeks reference period on which SMP is calculated to reduce the impact of salary sacrifice on your level of SMP."
SMP is calculated on your post salary sacrifice salary. Some women therefore want to stop the vouchers for the 8 week reference period (which runs from about week 17-25 of your pregnancy) used to calculate SMP to boost their salary during that time and make their SMP as high as possible. For most women, this would only affect the 6 weeks you get at 90%, so it's often not worth the hassle/doesn't work out particularly beneficial. However, if your salary after the sacrifice is low enough that you wouldn't qualify for SMP, stopping the vouchers for the few weeks when SMP is calculated can make a real difference.
"Once you start receiving Statutory Maternity Pay, it will not be regarded as a qualifying salary from which a sacrifice can be made. Therefore, if you are receiving any form of Statutory Maternity Pay you will not be able to exchange it for childcare vouchers."
This is true, as far as it goes. SMP is not salary you can swap for vouchers. For example, you might want to start your first child off in nursery a few weeks before you came back to work. If you were receiving SMP during this time, you couldn't sacrifice that SMP to buy childcare vouchers, because you cannot pay for them out of SMP. You have to pay for them out of salary. However, once you have done the salary sacrifice (for example, you are back at work after your first child, are using vouchers and then want to go off again), the best analysis is that the vouchers become a benefit and, because benefits have to continue during maternity leave, you keep getting them even though your employer cannot deduct the money from them from SMP (but, as I have mentioned, probably can from company enhanced maternity pay). Effectively, it means that some lucky women get the vouchers for free during second and subsequent pregnancies!
Is that any clearer?