When a player calls a tile and places it on the flat part of their rack or exposes tiles from their rack, they are committed to taking that discard.
Joker exchanges are done by first announcing a Joker exchange is to be made and then by handing the tile to the receiving player.
Jokers exposed before a hand is declared dead are still available for a Joker exchange.
Once a player has begun their turn by picking a tile from the wall, they may do a Joker exchange or multiple Joker exchanges from any player’s rack including their own.
If you claim a discard to start your turn, you must be able to complete an exposure with the discarded tile.
Discarded tiles cannot be claimed to be used in a joker exchange.