Hey Bob, don't you think it odd that I can get a trip/strike on you pretty much any time I want, but only the once per sparring session? It's like there is something that resets when we break for water or something. No matter how I try I can't get the second one to work right, but the first one is always flawless. Weird.
Let me contrast with this.
In-character: "Ok, you're a pretty big dude! Time to knock you down a peg, Spinning Hurricane Slash!"
Player: "Ok, using my encounter power, 18 vs AC, 11 damage, and he's knocked prone."
In-character: "Ouch, looks like he took a nasty fall when I hit him, probably bruised up in that armor he's wearing."
In-character: "Uh-oh, hear come your two cronies. Let's see how if they'd like a taste of what you got. Spinning Hurricane Slash!"
Player: "Ok, I want to knock him down next to his boss, that'll give my rogue friend Logan here combat advantage against them. Can I do a Athletics check to knock him prone?"
DM: "Sure, makes sense as an Athletics check. DC 16 on that, but no damage."
Player: "Cool. Got...another 18."
In-character: "And on your butt you go! Ladies and Gentlemen, Bowling for Hobgoblins!"
In-character: "OK, my cleric buddy just finished off your pal with some "mace" in the face. Time to die, pal, Reverse Spinning Hurricane Slash!.
Player: "He needs to go down, so I'll do a basic attack. 20 versus AC, 14 damage"
DM: "Ok, that drops him to neg 2. Do you kill him?"
Player: "We're on top of the tower, right?"
DM: "Sure are."
Player: "I'll give him a chance, and Hurricane Slash him off the top of the tower. Maybe he knows how to fly."
DM: "<Pause> Does bouncing count?"