Vrddhi in sigmatic aorist

A question to Piotr.
In your exposé on the PIE verb
(www.geocities.com/caraculiambro/Caraculiambro/Verbs.html) you write:

'For example, the root *bHer- `carry' is inherently durative, so the
preterite *bhér-e-t (corresponding to the present *bHér-e-ti `he
carries') means `he was carrying'. To express a completed action, one
would use a specially marked aorist form, in this case *bHé:r-s-t `he
carried, lifted' (a so-called sigmatic aorist). ... gWHen- ... aorist
*gWHé:n-s-.'

You don't express your opinion on the reasons of the root vowel
lenghthening in the sigmatic aorist 3sg. Merely phonetic lenghthening
of a stressed /e/ before a sonorant or glide in closed syllables?
Additional morphonemic marker of the sigmatic aorist? Anything else?

Sergei