The Rule of One states that when there is just one trump outstanding which is a master, you should normally leave it out. Why waste two of your trumps to get rid of a trump that will win anyway? As declarer, the Rule of One position (one master trump outstanding) is very powerful.

a) Dummy ♠ K632		b) Dummy ♠ 8632
--------------	♠ trumps	-------------
Declarer ♠ A754		Declarer ♠ A754

In (a), cash ♠AK then (assuming the suit splits 3-2) leave their master trump outstanding.

In (b), to reach the Rule of One position, duck the first spade. On regaining the lead, cash ♠A and (assuming a 3-2 split) leave their master trump outstanding. Note if you play ♠A and another, they can win and draw a third spade which you don’t want.

c) Dummy ♠ 4 3 2 ♥ A K 2		d) Dummy ♠ K 3 2 ♥ A 6 2
----------------	♠ trumps	---------------
Declarer ♠ A 8 7 6 5 ♥ Q 5 4 3		Declarer ♠ A 7 6 5 4 ♥ K 5 4 3

In (c) and (d), the aim is to ruff the fourth heart in the dummy, necessary unless hearts split 3-3. In (c), duck a spade. On regaining the lead, cash ♠A and, leaving their master trump outstanding, play ♥AKQ and ruff ♥5 with ♠4. Note you do not mind if a top heart is ruffed with their master spade as you can peacefully ruff ♥5. In (d), you must duck a heart first. On regaining the lead, you cash ♠AK to reach the Rule of One position. Then play ♥AK and ruff ♥5 with ♠3. Note if you instead play ♥AK and lose the third heart, they may lead a fourth heart with their partner ruffing from a doubleton spade — which you don’t want.

There may be another way: