Ill use excerpts from this site to answer your questions: sistersofcarmel.com/brown-scapular-information.php
1st. In answer to your question asking how the Scapular is devotion:
True devotion to the Blessed Virgin Mary consists in three things: VENERATION, CONFIDENCE AND LOVE. By simply wearing the Scapular, we can tell her every moment of the day that we venerate her, love her and trust in her protection. ...
The Scapular Is a Silent Prayer As Our Lord taught us to say the Our Father, Our Blessed Mother taught us the value of the scapular. When we use it as a prayer, Our Lady draws us to the Sacred Heart of Her Divine Son. It is good, therefore, to hold the scapular in the hand. A prayer offered while holding the Scapular is as perfect as a prayer can be. It is especially in time of temptation that we need the powerful intercession of God’s Mother. The evil spirit is utterly powerless when the wearer of a scapular faces temptation, calling upon the Holy Virgin in this silent devotion. “If you had recommended yourself to me, you would not have run into such danger,” was Our Lady’s gentle reproach to Blessed Alan de la Roche, one of her devoted servants.
2nd. In answer to your question regarding the promises:
The Sabbatine Privilege
The Blessed Virgin of Mount Carmel has promised to save those who wear the scapular from the fires of hell; She will also shorten their stay in purgatory if they should pass from this world still owing some debt of punishment.
This promise is found in a Bull of Pope John XXII. The Blessed Virgin appeared to him and, speaking of those who wear the Brown Scapular, said, “I, the Mother of Grace, shall descend on the Saturday after their death and whomsoever I shall find in purgatory I shall free so that I may lead them to the holy mountain of life everlasting.”
The Blessed Virgin assigned certain conditions which must be fulfilled:
Wear the Brown Scapular continuously.
Observe chastity according to one’s state in life (married/single).
Recite daily the Little Office of the Blessed Virgin OR Observe the fasts of the Church together with abstaining from meat on Wednesdays and Saturdays OR With permission of a priest, say five decades of Our Lady’s Most Holy Rosary OR With permission of a priest, substitute some other good work.
Pope Benedict XV, the celebrated World War I Pontiff, granted 500 days indulgence for devoutly kissing your scapular.
3rd. In regards to your question asking if you can take the Scapular off for sports and still enjoy the promises: I wear the Scapular during track workouts but it sometimes gets in my way. The Priest that invested me in the Scapular told me that my Scapular medal would suffice if I needed to take off the Scapular. It is part of my 4 way medal and very small.Even though i have used mine during sports and workouts, it hasn't gotten too bad. If it starts to fray, just sew it back together with some brown thread. (i think this is okay) Another option i have not done much looking into is the Athlete Scapular like the one here: carmeliteshoppe.com/content-product_info/product_id-1935/sport_or_crusader_scapular.html
You should also be invested sometime soon after you start wearing the Scapular if you choose to use the Scapular as a devotional
PM me if you have any questions.