The trochaic inversion

[fastmarshmallow]

I'm really trying to wrap my head around this. It's seductively frustrating. 

Before I attempt my own, can someone check if I'm interpreting this correctly?

dah DUM (IAMB) dah DUM (IAMB) dah DUM (IAMB) dah DUM (IAMB)

First feet is inverted -->

DUM dah (TROCHEE) dah DUM (IAMB) dah DUM (IAMB) dah DUM (IAMB)


So if I were to apply this to the opening line of Frost's Birches:

When I see birches bend to left and right

WHENi (TROCHEE) seeBIR (IAMB) chesBEND (IAMB) toLEFT (IAMB) andRIGHT (IAMB)
DUM dah dah DUM dah DUM dah DUM dah DUM  

So am I right in saying there is a trochaic inversion in the first feet? Stress on the first 'WHEN' and then I read 'I see' in unstressed, rapid succession, before the stress lands again on "BIR-".


I also have another question. What is it called when the line is in trochaic meter and then there is an inversion to an iamb i.e., basically the other way around? See below:

DUM dah DUM dah DUM dah 
dah DUM DUM dah DUM dah

It doesn't really count as, say, a 'spondaic substitution' - or does it? Because a spondee needs to be contained to one foot right i.e., DUM DUM dah DUM?