It's a secure way of showing others who you really are. Unlike photos or other videos, a Gestya can't be reused, faked or pre-recorded. Each time a Gestya is recorded, the account holder is asked to perform a series of unpredictable challenges, which are recorded for others to see later.
As the challenges are not known in advance, it's unfeasible to prerecord millions of possible Gestya combinations that may be requested.
Bogus Gestya's can be easily identified when the person in the clip isn't performing the right challenges or the video appears tampered. Suspicious clips can then be reported by viewers to warn others of potential identity issues.
No, and no one can use your clip as you need to approve each web service that wants to show your Gestya.
Getting others to stand in for someone else is tricky. But Gestya can be also be requested at later times to make it even harder to get the same stand in.
Gestya's can be played on any HTML5 capable browser, but it requires WebRTC to record. WebRTC is currently only supported on Chrome and Firefox. Hopefully support coming soon for IE and Safari (come on Microsoft & Apple!).