im not going to bother digging my sklansky out because surely there is a more obvious reason why it is better to double up at start if my tired, cold-infected brain is actually working properly. It pays to double up because:-
(i know it would obviously not work as ideally as this but.....)
First Hand of Rebuy: 1) You fold your junk
2) Player x and y (both with starting 2000 chips go all in)
3)For sake of it Player x wins and doubles up and with blinds now has at minimum 4020 chips)
Second Hand of Rebuy: 1) You look down at premium hand eg. 10 10,JJ,QQ,KK,AA,AKs *or at least an all in hand
2) Player x from previous hand (with 4020 chips) comes over the top all in.
3) You call, have him dominated and double up.
Now this is the important part:If you did not double up at the start you have won just 2000 chips off player x from a possible 4000!
Surely enough reason for doubling up at start.
Because as long as you find your premium hand after the very first hand there is going to be someone on your table with more than the starting hand (anything up to 4000 chips) giving you maximum value from your hand. (max- doubling to 8000)
If you start with 2000 chips you are surley only getting ~50% value in your first double up meaning you have to find another suitable hand to do reach the possible value of starting with 4000!
I hope that makes sense.
I even started to confuse myself by the end!!!

KingPoker
