XYZ is in effect whenever our side has made 3 (or more) bids at the one level, and it’s Responder’s turn to bid:
1♣️-1♦️-1❤️, 1♣️-1♦️-1♠️, 1♣️-1♦️-1NT, 1♣️-1❤️-1♠️, 1♣️-1❤️-1NT, 1♣️-1♠️-1NT, 1♦️-1❤️-1♠️, 1♦️-1❤️-1NT, 1♦️-1♠️-1NT, 1❤️-1♠️-1NT, or even 1♣️-1♦️-1❤️-1♠️-1NT.

• Any direct bid on the two-level is a sign-off.
• 2♣️ by the responder is invitational (or weak on diamonds), asking the opener to bid 2♦️.
• 1♦️-1❤️-1NT-2NT is a relay(!) to sign-off in clubs.
• 1♦️-1❤️-1NT-2♣️-2♦️-2NT is an invitation to 3NT.
• 2♦️ by the responder is game forcing. This artificially tells your partner that you have 13+ points.
• With less than invitational values, Responder may rebid a 5+ card major suit to play.
• When Responder has more than enough for the game and a long, strong 6+ card suit and is considering a slam, he does not use the 2♦️ game force. Instead, he jumps to the 3 level in his suit.
• XYZ replaces two-level new minor forcing.
• XYZ is ON when opponents interfere.

Reference: bridgewebs.com slides and bridgehands.com post