Why do they dwindle the blood pressure surrounded by the arm when takiing an injection?

well I've see many movies surrounded by which they tie the arm up before getting a syringe surrounded by them so I was only just curiousWhy do they dwindle the blood pressure surrounded by the arm when takiing an injection?
Because when the blood flow is restricted, the vein swells, making it easier for the "injector" to find the capillary.
They are stopping the blood flow in the vein so that they will fill up making it easier to do the injection
Its to block blood flow so the viens will buldge, (so they can find them)to inject what ever they are injecting.
By tying the arm until that time inserting the needle, the blood pressure in reality increases because venous flow is blocked/restricted while arterial flow is still pushing against it. I am a Respiratory Therapist and I have not drawn venous blood that channel. I have drawn oodles arterial blood gases and the arterial pressure (which is sophisticated than venous pressure) pumps blood into the syringe when the needle is within the blood vessel. It "flashes" to let you know that you if truth be told hit blood.
One important entry to remember is that movies are fiction. I have watch many tv shows and movies where on earth things are being shown contained by a horribly inaccurate attitude. (ie. intubated people chitchat, people going from run of the mill sinus rhythm to flatline, etc...)
The torniquet is used to decrease venous return and plague the veins below the even of the torniquet. That way you can in truth get the entry in in that. Then the torniquet is released, because the point of the injection is to get it into the (temporarily blocked) venous circulation so it'll return with from there into the system.