Training lets you suspend the Digimon during your main phase to tuck the top card of your deck under it, and it works even in the breeding area. On its own that looks slow, but stacked digivolution cards feed everything from Fragment to "when digivolving" payoffs, so Training is how you build a fat stack before the game punishes you for it. Growing in the breeding area is the safest version: nobody can touch you there.
16-41-1. <Training> is a keyword effect that reads "By suspending this Digimon during the main phase, place the top card of your deck at the bottom of this Digimon's digivolution cards. This effect can also activate in the breeding area." 16-41-2. <Training> is an activation-type effect. (For details, refer to 15-8-4 "Activation-Type Effects") 16-41-3. "By suspending this Digimon" in <Training> is an optional processing condition. If this condition is performed, "place the top card of your deck at the bottom of this Digimon's digivolution cards" is mandatory processing.
Comprehensive Rules §16-41 — official rules page
20 cards carry this keyword:
Rule text © Bandai, quoted from the official Comprehensive Rules for reference. Combalo is not affiliated with Bandai.
