...if the arms are allowed to extend and "lock" into position they can be difficult-to-impossible to move depending on the relative strength of the practitioners.
More muscle groups (Shoulder, Chest, Back and possibly even the biceps to a lesser degree) involved for the guy trying to hold his arms in as oppossed to the guy trying to push out (mostly just the shoulders for those who don't "index", brace or anchor properly.
However, when the arms are in motion (meaning still in the act of extending into the push) they are very pliable as it is a diffrent set of mechanisms involved in the body moving itself as opposed to the body stabilizing itself and in most (read: 99.9%) cases these mechanisms CANNOT occur simultaneously for the same muscle/joint groups.
"Timing is the biggest basic." -- Mr. Planas (as told to me by Max)
Ride the push and you can part his arms with the added benefit of loading your weapons while unloading his. Start to early and you unload your own weapons over extending. Start too late and you hit the "stabilizing phase" of his movement which gives him the bio-mechanical advantage and he "ain't gonna move".
Good topic. Always good to study structure, function, leverage, muscle groups and most importantly..timing.
James Hawkins III, SI
Hawkins Kenpo Ju Jitsu