Overload is synonymous with overdose and excess. It may also, in this context, be synonymous with pain and hurt.

As long as you do not perform voluntarily, or get involved involuntarily, in these activities too frequently then they can do you no long term damage, but if you suffer them or perform them frequently you may well cause yourself damage - physical damage - pain and hurt. 

Overload activities are often very effective at providing spiritual experience, but they are also potentially more risky whether you do them deliberately or suffer them involuntarily.

What is too much depends on what it is we are using.  Vitamin A in the quantities found in food is not overdose.  But take it as a supplement or have carrot juice in vast quantities and it is overdose.  It becomes a poison.

No natural substances are inherently poisonous, no activities are inherently hurtful, it is when they are taken to excess that they become so - when sadness turns to depression or uncontrollable grief, when being alone turns to desperate loneliness.  So all the activities in this section work via excess, hopefully temporary excess. 

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