Prima custodia is another example of a custodia, that may be used by itself as an obsessio. The possibilities of prima custodia seem to be limited - it can besiege only another prima custodia. The idea is that you can extend both your hands from prima custodia, hitting the opponent's shield before he can move it, thus pinning it down at his body. As the first play explains, this kind of siege can be defended by a halpschlit-like descending cut from prima custodia, that will threaten the besieger and fend the attempt at pinning the shield down.
Because prima custodia is a first position you will enter when drawing your weapons, it is possible that these sieges and counterattacks are in fact happening in iaido-style quick attacks launched from sheathed weapons.