I must defer to spiphany's explanation. She* made the distinction clearly and accurately.
pster wrote:My only remaining question would be how Nate or anybody determines when the stem contains the vowel and when it doesn't.
If I'm not sure, I usually look in Smyth's list of verbs or in LSJ.
With these particular 2nd perfect verbs, I knew by memory the way the stem should be.
λιπ- (true root, 2nd aorist, no σ) turns to λειπ- (present/imperfect tense; λειψ- in future)
turns to λε-λοιπα- (2nd perfect, no κ).
στα- (present, no thematic vowel ε/ο; after reduplication and loss of initial sigma
ἵ-στη- in sg. act. ind., ἵ-στα- in pl.) turns to στησ- (future) turns to στησα- (1st aorist) and
στη- (athematic 2nd aorist act., no σ) turns to ἑ-στηκα- (1st perfect) and ἑ-στα- (athematic
2nd perfect, no κ).
* Edited. My apologies.