The more internal pressure is applied to this join, the more the o-ring is distorted inside its groove, which can improve its overall sealing force up to a point - but beyond a certain pressure, or under more dynamic workloads, this can cause failure of the seal. It's important to get the balance right between o-ring material, size and working environment in order to fulfil the role you need it to perform.
http://www.yuefeng-seal.com/o-rings/nbr-o-ring/
