An exit tax is a sale you did not make, charged at a price the market may not hold. Hold 100 of basis in a jurisdiction that charges 25% on unrealized gains when you leave, with the asset at 300, and the bill is 50. Settling that from the position sells a sixth of the holding; the basis steps up to 300 and the claim ends.

Halve the price to 150 and the remaining five sixths are worth 125, against a stepped-up basis of 250. The 50 already paid is four times the 12.50 the same holding would owe at that price. What comes back is a capital loss, deductible against gains that may not arrive.