Crosstalk is a conversation where each person speaks their native language. You speak your native language; your partner speaks the target language. This lets you focus entirely on understanding, without the stress of producing the language yourself.
Find a native speaker of your target language who is learning your native language. Have a normal conversation, but each person speaks the language they're most comfortable with. You speak English (or whatever your native language is), they speak the target language.
It sounds strange, but it works. Because both people are speaking comfortably, the conversation can be genuinely interesting — much more so than the stilted exchanges you'd have if both of you were struggling in your second languages.
Conversations are two sided: you must speak and listen. Trying to do both while you're still a beginner is very difficult.
Crosstalk trains the comprehension side of conversation. In a real conversation, you need to understand someone speaking directly to you, at natural speed, about unpredictable topics. Crosstalk gives you practice with exactly this, without the added cognitive load of speaking.
Then, once you're comfortable with understanding, adding in speaking is much easier.
It also has social benefits. You can make real friends through crosstalk very early on in your learning journey, because the conversations are enjoyable for both people from the start.
Look for native speakers of your target language who are learning your native language. Language exchange apps (Tandem, HelloTalk) and online communities are the easiest places to find them. If you have family members who speak the target language, crosstalk can feel very natural.
Crosstalk is introduced in Phase 3D as preparation for real conversation in Phase 4, but you can start it as early as you want! It's especially powerful if you're learning the language of family members or close friends.