Nosebands

drop doesn't need to be tight to be effective. as long as it's correctly fitted, i.e. 4 fingers' width above top of nostril, the angle it sits at makes it effective. therefore the horse can still flex its jaw (which it needs to be able to do), swallow etc easily. drop works better with certain bits though - e.g. i usually use full cheek or fulmer cheeks with the drop because it all stays nicely in place then. if you use a drop with, say, a hanging cheek, it rides up it and doesn't stay still.
i'm not a fan of flashes, so will leave someone else to explain about them! their action is far more obvious, and they need to be tighter to be effective.
 
I think A flash should be tighter to be effective but my pet hate is seeing a tight flash strap with a loose cavesson, it just pulls the front of the cavesson down,which makes it pointless, if fitted properly the cavesson and flash should be the same tension or the flash strap looser. The action is obviouly to try and prevent the horse from opening it's mouth and crossing it's jaw to an extent, personally i think it is nothing a good wide crank/sinch noseband can't do. Remember it is just as important for a noseband not being too low as not being too high, feel down the side of the nose, as kerilli said about four fingers from the nostrils the bone at the side of the nasal cavities stops and the bone on the front graduates to cartilage. A tight noseband on this nasal peak can compress it and in sever cases break the nasal peak.
 
Well, the flash was designed purely so that people could use a drop with a standing martingale. I don't like either of them as I think they must be uncomfortable inside the mouth (squashing the inner cheek onto the teeth) and possible restricting breathing (especially a drop). I prefer a high ring grackle if I need to use a drop action as it sits over the cheekbone.

A drop simply closes the mouth lower down, a flash higher up and in 2 places thus stronger in action.
 
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