What exactly does Attach() do in Entity Framework? [duplicate]

Just as a point of interest the code you have posted does nothing

using (var context = new MyEntities())
{
    context.Attach(client);
    context.SaveChanges();
}

All this does is attach the entity to the tracking graph make no modifications to the entity and save it.

Any changes made to the object before attach are ignored in the save

What would be more interesting is if it actually updated a property ie:

using (var context = new MyEntities())
{
    context.Attach(client);
    client.Name = "Bob";
    context.SaveChanges();
}