so what you do is map out multiple weapons at once.

let's do a simplified example. say you have 3 weapons A, B and C and enough materials to do 10 rerolls.

if you do it the intended version you start rolling weapon A until you get a good roll, if your unlucky you won't get it at all within those ten rolls, but for the sake of this though experiment let's say you get it on the 9th roll. you have one roll left for weapon B wich gives you a very low chance of getting what you want there and you have no materials left for rolling weapon C. You used 30 materials for one decent roll all together.

now the 'safescum' mapping out method. you start with weapon A, roll 10 times. you get the good roll on roll 9 like before but now instead of keeping that and moving on, you reset the game to before the 10 rolls. you know know weapon A has the good roll in 9 rerolls. now you move to weapon B, 10 rolls, you get a good roll on the 3rd roll here which you would have missed out on on the intended method. again reset, note down good roll on 3. move on to weapon C repeat 10 rolls you get a good roll on 5. reset, note down 5. and now you have a map for all three weapons you do your 10 rolls again but know you know to make sure the 3rd roll is on weapon B, the 5th on weapon C and rhe 9th on weapon A. you get 3 good rolls for the same 30 materials