This is an efficient swarm engine using [BT1-056] Petermon. At 5-cost he can revive or play Tinkermon, which is very memory-efficient. Assuming cheapest hard-play LV3 digimons are always played at 2-cost, Petermon is effectively a 3-cost LV4 digimon that revives (does not cost your hand size).
While there are other digimons (especially in violet/purple) that can revive very efficiently, this is the only revival engine in yellow. Furthermore, Petermon is the only LV4 digimon with a revival ability, other digimons in violet are LV6.
[BT3-090] Mastemon can revive Petermon, which in turn revives Tinkermon. At 4-cost, this becomes the cheapest method to summon 3 digimons onto the field!