Brace position - Why hands atop head in the UK, but on ankles or atop seat in front in the US? Source: Does the brace position save lives?,  By Oliver Smith, 2013 Mar 13, telegraph.co.uk

&quot;To adopt the brace position passengers should place their feet and knees together with their feet firmly on the floor,” added [Steve Allright, a British Airways training captain] .... “Feet should be flat or on the balls of their feet, and further back than the knees. In the event of an impact, this position helps prevent shins and legs being broken against the base of the seat in front, which would obviously hinder evacuation.
&quot;Passengers should bend as far forward as possible, if it's in reach, they should rest their head against the seat in front, with hands placed on the back of their head. Hands should be placed on top of one another (not with fingers interlocked) [emphasis mine; this seems a critical fact], elbows should be tucked in to their sides. The head should be as far below the top of the seats as possible. This position prevents flailing of the arms, minimises the risk of broken fingers and protects the head from moving objects.”
The position varies slightly in the US. Rather than placing hands on the back of the head, passengers are advised to place them on top of the seat in front, or to hold their ankles.

1. Why the difference in the hands' position? Which is more protective?
2. What does it mean to 'hold their ankles' with the hands? Simply place your hands atop your ankles?
The difference is just because there are often disputed methods on which technique is best. To be brutally honest, many accidents are so severe and can't really be analysed, so this is still a point of contention.

I would hypothesize that the UK version is safer as the hand position protects the skull and face from direct impact with the seat in front; instead colliding with the inside forearm. This will naturally reduce impulse (strengthy plastic vs. fleshy skin) and should thus reduce injuries.

The concept of holding ankles stems from the position your body is put in when holding your ankles. Any kind of "holding" is fine, touching on top or sides. It puts your body into a position that ducks your head and should (because of close seat proximity) rest your head against the seat infront anyway.