For those who don't know, it's a sandbag. Basically, it's a duffle bag filled with smaller bags of sand and it's used for various strength and conditioning exercises. Before everyone presumes my boyfriend is the most unromantic sod in the world, I have to stress that this is what I asked him for and I was very pleased to receive it.
Sandbags can be used for a variety of exercises, depending on how heavy the bag is. My sandbag is adjustable by removing and adding more bags of sand. I currently have 60kg of sand (roughly bodyweight) but the duffle bag is apparently loadable up to several hundred pounds. The big difference between a sandbag and a barbell is that the sand moves: it's not equally distributed and it's difficult to hold onto so it can challenge strength, grip and stability at the same time. A barbell is usually a nice, evenly-weighted equilibrium with a pleasant 1'' knurled handle and rotating ends. As well as challenging the phasic (moving) muscles of the body, a sandbag can also challenge the smaller, stabilising muscles as well as the core.
I will spend next week experimenting with the sandbag and working out how I can integrate it into my routine. Initially I will try shouldering it, squatting and lunging with it, Turkish get-ups and loading it onto a platform (like an atlas stone). I'm going to try to avoid gripping the fabric too much as my grip takes quite a beating from the combination of BJJ and work.