Freedom of spirit.
It's a gift. I think an 'ordinary mystical' experience of it is common after making a good confession. That peace and joy one experiences after absolution, the grace which expands the heart in charity and gratitude. When the Holy Father talks about these things, I think he echoes the Psalmist:
I wonder if this is what the Holy Father means when he speaks against rigidity?
I will run the way of your commands;
you give freedom to my heart. - Ps. 119: 32
33 Teach me the demands of your statutes
and I will keep them to the end.
34 Train me to observe your law,
to keep it with my heart.
35 Guide me in the path of your commands;
for there is my delight.
36 Bend my heart to your will
and not to love of gain.
37 Keep my eyes from what is false;
by your word, give me life.
38 Keep the promise you have made
to the servant who fears you.
39 Keep me from the scorn I dread,
for your decrees are good.
40 See, I long for your precepts;
then in your justice, give me life.
41 Lord, let your love come upon me,
the saving help of your promise.
42 And I shall answer those who taunt me
for I trust in your word.