Those elastic black stretching just behind the girth area to protect the horse from spur rubs.
Lots of controversy surround belly band, yet they have quickly risen in popularity in the show jumping world (as they are banned in dressage land). So why would you use one? Some claim to have sensitive horses which get spur marks from the tiniest touch of the spur. Other say they want the action of the spur without it being so severe to the horse. It’s the new neoprene altenrative to leaving unclipped areas right on the area where the rider uses his/her leg. While some of this may seem convincing enough, it is more often than not used to hide other problems.
So should belly bands ever be used on a horse? The answer is no, at least not until there are no option left. It will take a long time for there to not be any other options. If you find yourself spurring and spurring, a reevaluation of your leg aids is needed. Ask yourself, are you taking off your leg properly? Some riders have a problem with always gripping with their calves, which makes the horse numb to the pressure. So yes, spurs should never be given to riders without an independent leg/seat. Beyond correct use of the leg, with have problems with tack, teeth, body, legs, etc. Have you worked those out?
A belly band (just like the draw reina) isn’t something that has to be evil. However, it can easily become evil if put into the wrong hands (and sometimes this is unintentional). With horses (and actually most things in life) it is best to start with the cause and work from there. Remember, less is better than more!