I found a good answer in the really good Spades book Master Spades by Steve Fleishman, his answer is the following:
Lead the second-lowest card in a safe suit. This signals to partner that this suit is safe and that you have one lower card.
Why this is good: when opponents play a low card in this suit, because partner knows you have another low card he don't have to cover and can save his high card for next trick.
This convention also lets you inform your partner if you hold a low singlton, by playing the 2 (or the 3 when the 2 either held by partner or will be played) partners know that you don't have a lower card in that suit, hence you had a singlton. Now partner will lead that suit on the first oppratonity, so you can discarde a high card in a diffrent suit.