When a part is grabbed, the part is docked to the eva part, the fixed joint are removed and the grabbed part position are constantly updated related to the eva.
This as been done like this because it's the only way to not disrupt the eva RCS in space. (It get mad with a part physically attached, at some point, the kerbal start spinning and you completly lost control)

However, that will be nice to find a solution for keeping the fixed joint and let's the physic handle the grabbed part directly. It will allow physic interaction of the part with the eva, so jetpacks, parachutes, etc... will be possible

For now an option exist in the .cfg to activate it.
Maybe let this option toggleable in game when needed ?

Well, certainly don't have the physics part on by default until the game has multi-core physics enabled. Otherwise you're just stressing out an engine incapable of handling the tiniest of anything. -system: Ohgod, something to do with physics, kraken, help me! universeimplodes-