The yeast do all their work before oven entry by filling the dough with CO2 bubbles. Oven spring is just the expansion of the already present CO2 (as both gas and dissolved in the liquid). The dough springs only if it is weak enough not to resist and the internal pressure is adequate to inflate it.

You can do a neat experiment if you have a Save-A-Meal or some other vacuum sealer: proof 100g of dough in a clear straight sided 12 oz water glass; pull a good vacuum and measure how much it expands. The volume grown should approximate your potential oven spring. If you bake a similarly proofed piece of dough in your oven and get more oven spring, it can be attribute the overage to yeast action after oven entry. If you get less, the dough got too strong too fast relative to how fast the internal pressure was building up. This is generally because your oven is too cool and you cook the outside before the dissolved CO2 has turned to gas and really pushed the oven spring.

For real oven conditions, measurable yeast activity after oven entry is a wives tale and total BS! (IMHO)

Doc