During the last part of a concentration exercise (focusing eyes on a point between eyebrows) I felt a need to contract my Pubococcygeus muscle. During contraction I was feeling some vibration in my left wrist. What was that? It was nice vibration while my mind was clear.
DISCLAIMER: It isn't that such phenomenons don't occur during Buddhist meditation, but typically such bodily phenomenons are not emphasized in most Buddhist traditions, and thus Buddhists lack the vocabulary. Tibetan Tantra practice is an exception, but that's because it derives a lot of its vocabulary and practices from tantric yoga. The subtle body is viewed in Buddhism as personified impermanence.
From your description it appears to be a spontaneous Mula Bhandha. Like sneezing or coughing, spontaneous yogic postures are natural bodily occurrences, and are nothing to be wary of. They are triggered by conditions. For example, we sneeze in response to being in a room full of dust, similarly, when there are certain subtle energy manifestations in the body that are in need of correction, the body makes use of such locks or yoga mudras.
Image source: http://sequencewiz.org/2014/09/10/mula-bandha/
Without knowing more details about the quality and nature of vibration you felt, it would be hard to say for sure, but it sounds like Piti / Sukha. The clear mind you mention is likely the mind born of seclusion.
I'd recommend gaining reasonable perfection in either Hatha Yoga meditation, or in Buddhist meditation. Unless you have strong reasons for doing so, dabbling with both at the same time can be unproductive.