I found a nice piece on "active participation" in Denis Crouan’s The History and the Future of the Roman Liturgy (2001, in English from Ignatius Press, 2005, p. 279 – my emphasis):
Does Gregorian chat prevent the active participation of the faithful?
The expression used in the Constitution Sacrosanctum Concilium is "participatio actuosa" and not "participatio activa". Vatican II called for an "effective" participation in the liturgy, that is to say, a participation that is in contrast to a "passive participation" as well as to an "activist participation".
The true and only participation that the Church wants is the kind that results from an interior attitude that places us in a state of receptivity for the liturgy.
In this, too, it is obvious that the virtues of Gregorian chant shield us against a sort of participation in the liturgy that is too dependent upon contemporary mindsets and would end up being no more than a sterile activism.
Gregorian chant safeguards us against that ceaseless agitation that seems to have taken over a great number of contemporary Mass and that ends up making the liturgical space a sort of experimental laboratory run exclusively by those who mistake the Church for an "international volunteer association" [This is an expression used by a bishop during the Synod of the Bishops of Europe that was held at the Vatican in 1999.].
Back in August, in Camden, NJ, I put forward this same (correct) vision. Here is a snippet:
It is a hard fact of our fallen human state that we can be either deluded activists or passive spectators at any Mass, “Tridentine” Mass or Novus Ordo, in English or in Latin, no matter how diverting or engaging it is made. When I hear the claim that if people aren’t allowed to sing everything and move things around, they are being turned into passive spectators, I respond that it is entirely possible to sing and be busy doing things and have your mind be a thousand miles away. Have you ever caught yourself singing, whistling, humming, doing things like gardening, driving, or even reading when you suddenly realize that you have turned several shovels full, street corners or pages, and have not the slightest recollection of what you just did? You can sing every verse of every hymn and all along be thinking about the groceries you have to buy. You can carry things, stand up and kneel down, and really be participating far less than someone who is sitting still in the pew, who cannot stand or kneel, cannot see the sanctuary clearly or very easily hear the prayers or sermon. And yet, and yet, with every breath and heartbeat, he knows why he is there; that person senses the Real Presence, and longs to be a part of what is taking place. Active participation is made possible by baptism and by our willed, conscious, active interior union with the action of the Mass and the true Actor, Jesus Christ. Attentive watching and careful listening are not easy, friends. Effort and practice are needed to get past the distractions.
… there are those moments in Mass when we are called upon to participate actively by receiving and then, on the foundation of our full, conscious and active interior receptivity, we use our voice and gestures in a way that is far more beneficial by the fact we have first received.