The phrase "to use the shield" means to protect yourself or others from harm or danger. It's like holding up a protective barrier to keep something or someone safe. Full definition
On average, the mothers used the shield for about 24 % of their time breastfeeding. (breastfeedingtoday-llli.org)
A mother can feel at ease about using the shield as long as it helps the baby breastfeed more effectively. (breastfeedingtoday-llli.org)
I have the feeling that using the shields at the beginning made it much easier for my son to drink, probably because my nipples are very small. (justbreastfeeding.com)