The main ingredient is sodium bicarbonate which in its powdered form, along with the magnesium carbonate help to absorb odours. The blue appearance underneath is from the wax that's used to hold the bar together. The best way to apply it, so it stays on for a while is to gently dab the bar on your armpit, rather than rub or roll as you do with a conventional deodrant. I've also found that occasionally filing the bar, for example with a nail file produces more powder and is easier to apply. I'm not sure how I'd feel about this product if I had a lifestyle that included a lot of physical activity and exercise. Admittedly I never use for this gym because I don't have that much faith in it and want to avoid an embarrassing situation, and so do from time to time go back to a conventional anti-perspirant.
I'm really very happy with the way this deodrant works on a day to day basis and so will continue to use it as my everyday deodrant. I do feel like its necessary though for a bit of extra help sometimes so I'm not entirely against using an aluminium based deodrant and at least for the most part I'm avoiding putting harsh chemicals in sensitive areas. Another point to add about this product is it's durability. I'm not sure you could carry this bar around with you in your bag, its quite hard and sturdy but the powder does crumble away and I can imagine it crumbling much quicker if it was to get knocked about too much. I use this pretty much only once or twice a day, and only when I'm home, so there's no need to carry it about for me. It's lasted an incredibly long time too so has potentially saved me a little bit of money.