This hand is all about the power of shape. East overcalls quietly unsure how to value all their cards in the opponents bis suit. But when partner raises trumps East can predict a good chance that game might make and bid to 4S.
West
North
East
South
Pass
1H
1S
Pass
2S
Pass
4S
All pass
TIP:
Bid your hand as a conversation with partner. BUT in competition you are going to war and you do not want to communicate helpfully with the opposition. SO only contribute more if you have something particular and useful to say.
On this deal North has a perfectly normal 1H opening, but after EW enter the auction - NS should prepare to defend quietly as neither has anything more to tell each other.