Pair Hands (1)
Let us first think about whether you should aim for Chiitoitsu or Toitoi.
1. Six Pairs
Switching from a Chiitoitsu tenpai into Toitoi is a bad play.
Even if a tile you could pon gets discarded, you should stay still and riichi once the hand improves into a better tanki wait.
Example 1
If a hand like this reaches tenpai very early, that is a separate case.
But unless the shape is that extreme,
there is no reason to pon and push a hard-earned Chiitoitsu tenpai all the way back to two-shanten just to change into Toitoi.
Theory Summary
Do not force a Chiitoitsu tenpai
into Toitoi.
2. Five Pairs
Five pairs is exactly the branching point between the two routes.
If you only compare shanten numbers, Chiitoitsu is one-shanten, while Toitoi is still two-shanten even after a pon.
However, Chiitoitsu must rely on self-drawing one of the three remaining unmatched tiles,
so it is a hand that can take time to move from one-shanten to tenpai.
Depending on the hand, it can be better to step the shanten count back and switch to Toitoi.
Example 2
Dora
This is the kind of hand where calling and moving toward Toitoi is fine.
The reason is that there are many easy-to-pon tiles, and there is also a yakuhai pair.
Tiles that are easy to pon are simply tiles that are easy for other players to discard.
Terminals and honors, meaning 1s, 9s, and honor tiles, are easiest to call,
and 2s and 8s are next.
Middle tiles from 3 through 7 are often used inside players' hands,
so they are generally harder to pon.
That is also why Tanyao Toitoi is difficult to build.
Theory
Terminals and honors are easy to call.
Number tiles from 3 through 7 are harder to call.
Example 3
Dora
Example 3 should not be called.
Even if it works out, it will probably only end at 2600 points, and there are too many tiles that are hard to pon.
Pon 
Pon 
Cheap, hard to win, and with no defensive value.
You could call this a textbook example of a bad call.
Theory
If all of the following are true:
・you have pairs that are easy to call
・you can expect at least 3900 points
・the hand is still early
then it is fine to switch a Chiitoitsu one-shanten hand over to Toitoi!
Example 4
Dora 
What about Example 4?
The tiles are not especially easy to call, but I still think calling is fine here.
That is because even if this hand does not become Toitoi,
it still has a Tanyao route or a Hatsu-backup route.
So the idea is to pon first, cut ,
and then decide from the state of the hand whether to keep aiming at the 5200-point Toitoi
or to accept 1000 points instead.
Immediately throwing away is too extreme.
That kind of play is just forcing your own idea on the hand.
3. Four Pairs Plus One Concealed Triplet
The theory here is to take the shape as a Suuankou two-shanten hand.
Example 5
Tsumo
In other words, with a case like Example 5,
the basic idea is to cut one of , , or ,
take the six-tile Chiitoitsu one-shanten shape,
and if a pon-able tile appears, pon it and head toward Toitoi.
If you compare them only as one-shanten hands,
Toitoi reaches tenpai much more easily than Chiitoitsu,
and it also keeps open the possibility of Sanankou or even Suuankou,
so you should leave the Toitoi route alive.
Theory Summary
If you already have one concealed triplet,
keep both Chiitoitsu and Toitoi available,
and pon if a callable tile appears!
However, if two copies of one of the pair tiles are already visible in the pond,
meaning that pair can no longer become a triplet,
then you should cut and commit to Chiitoitsu.
There is no reason to chase an almost hopeless yakuman possibility.
Original Japanese page: http://beginners.biz/teyaku/teyaku09.html