Worked example
The top card is held at 10,000. You stake 12,000 and pay 13,800 (12,000 + 1,800). The 10,000 holder walks with 11,800.
Someone stakes 15,000 over you and pays 17,250. You walk with 12,000 + 2,250 = 14,250: up 450 on the 13,800 you paid.
If nobody had topped you before the clock ran out, you would take your 12,000 back: down the 1,800 you paid to bump.
