Kit, good points. Thanks.
Eric, all I have to go on is what has been publicly published when it comes to Hisa and the Takumakai. I would guess it means that the goal of Daito-ryu waza is to attack the joints (joint lock controls and/or dislocations) using the legs rather than the arms (which means attack the joints in the context of pinning techniques, since the use of legs against the joints would be easier).
The main emphasis of Daito-ryu techniques in recent generations, from my observation, is to control/pin the opponent, rather than to kill or seriously injure them. So we could presume that in recent generations attacking the joints means using the legs to apply joint locks. On the other hand, there is of course an allowance for the escalation of force should it become necessary, due to:
1) Incorrect, incomplete, or ineffective application of a controlling technique.
2) The seriousness of the situation (armed with a gun, bladed weapon, etc.).
3) Battlefield applications, modern or otherwise.
4) The threat of multiple opponents.
In these cases the legs may be incorporated to dislocate the joints, break bones, and apply finishing blows (ie: kicks, stomping their guts out, etc.).
In general, an escalation of force would include strong atemi to vital areas (causing serious bodily injury or death); chokes; dislocation/breaking of the joints or bones (disabling parts of their body to remove or reduce the likelihood or effectiveness of further attacks); concussing or crushing the head/neck through throws; stabs, cuts, or decapitations using bladed weapons. These are all understood to be options, but are not always taught explicitly (at least at all levels) in modern times.
Historically, we know that Sokaku used all of these methods at one time or another through the stories of his exploits and an analysis of his techniques.
During multi-person attacks, Sokaku used both evasive slipping movements combined with short-throws and/or atemi, or, entangled/piled the opponents at his feet in front of him, depending on the opportunities of the situation. For multi-person attacks involving evasive movement the goal would shift to damaging the opponent from a standing position due to the reduced time and opportunity to deal with each person - and of course, the tactic of taking opponents out of the mix upon first contact, rather than allowing them to attack repeatedly.
Furthermore, there are many other examples within Daito-ryu and Aikido where the arms or weapons are used for finishing blows and breaks rather than the legs. I could be completely wrong, but I suspect that pins with the arms are taught as basic pins, and pins with the legs are taught only after pins with the arms are mastered (within the Takumakai curriculum at least).
In any event, the idea of completely controlling of "attacking the joints" once the opponent is on the ground is actually consistent (for the most part) with what can be seen in other branches of Daito-ryu, including Aikido. In other branches, the standard atemi and pressure point attacks performed from the standing position are usually applied in order to distract the opponent (ie: backhand to the eyes) and/or weaken their strength so that a throw or control can be performed with minimal resistance. It is usually on the ground the opponent is then either pinned, damaged, struck, or killed. Other branches, including Aikido, also make use of the legs for atemi and joint attacks, but they definitely don't emphasize it as much as the Takumakai lines.
PS - There is a little more information on this subject in the Takumakai thread:
Daito-ryu Aikijujutsu Takumakai
Regards,