Back-up rings are not seals themselves, but are usually used in conjunction with O-rings in high-pressure applications.

备份环本身不是密封件，通常在高压应用中与O型圈配套使用。

youdao

Finally, however, Pompey's pressure told as the tireless Dindane nodded in at the back post from O'Hara's enticing deep centre.

BBC: Portsmouth 1-1 Sunderland

There will be immense pressure on scrum-half Tomas O'Leary but he will be helped by his Munster half-back partner Ronan O'Gara.

BBC: Jeremy Guscott column