A handpalm is a scenario where a normally-competitive situation has become so unbalanced that one side dominates all of the others and exercises an overwhelming degree of control. It metaphorically refers to the dominant player holding all the rest of the players "in the palm of the hand", meaning it can crush them and end the contest at any point. At this point, any actual competition purely symbolic - the contest continues solely at the desire of the dominant player.
A situation like this can arise in a simple game such as checkers or chess, or it can apply to more complex scenarios like economic competition, academic mastery of a given field, or military might.
Note that merely being the strongest player is not enough for a handpalm scenario. The scenario must become so lopsided that no action by any of the opposing elements can possibly break the victor's dominant position, short of intentional self-handicap or monumentally inept play.
HMB: "I played chess against my girlfriend last night. It was brutal. She started with a handicap of one Bishop, one Knight, and one Rook and she STILL managed to corner me with two Queens and a Rook. She then spent ten moves of her stalemate count just taunting me with them to make her point."
HDT: "Wow. I've never seen a handpalm of that magnitude. Rusty?"
HMB: "You're telling me. I played a game of chess against myself the other day and somehow managed to lose."
HMB: "My girlfriend inflicted the most intense handpalm last night. I'm still staggering from the blow."
DTM (who doesn't understand the term): "NIIICE! High five?"
HMB: "...no. No, I think not..."